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2006 Rx400h Max Tire Size Question


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Can someone definitively tell me what are the maximum size tires I can safely put on my 2006 RX400h? I drive my family in this car, so I don't want jeopardize our safety in any way.

Since you say safety is your concern I will assume you mean tread width....

No one can give you a definite answer to this but I can tell you the primary limiting factor is the tight clearance at the rear between the tire sidewall and the suspension components.

Personally I added 1.6" wheel spacers all around primarily so I would be able to use tire chains safely when the time came. At the same time I upgraded to 17X8 wheels and the appropreate +1" tires. Wider tread = more roadbed adhesion, wider stance = more stability against rollover.

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Can someone definitively tell me what are the maximum size tires I can safely put on my 2006 RX400h? I drive my family in this car, so I don't want jeopardize our safety in any way.

You can run 255/55VR18, 245/55VR18, or 255/50VR18 tires on your OEM wheels without any safety issues. Be sure the load rating is at least 99 and do not drop down to lower speed ratings like H. Some minor rubbing in hard cornering may be found for the larger of those, but if you are doing that with your family in the car... then you really aren't that concerned about safety are you? :ph34r:

Your safest bet if you don't want to take any risk is to just get the the OEM tire size. The Bridgestone Dueler H/L Alenza is your tire of choice for this size.

CAUTION: I would avoid wheel spacers as they change the way loads are transfered from the wheel to the hub. These OEM wheels are desinged to fit over a feature on the hub so that the wheel bolts are only in tension (no shear loading). If you install spacers, this function is lost and the wheel bolts now have to carry tension AND shear loads. They were not designed for this. This is a REAL safety issue.

To move to wider tires without any clearance issues or spacers you will need to get different rims that have a greater offset from the hub. Just be certain thay engage the shear feature on the hub in the same way the OEM wheels do, or you will be placing the botls in shear again.

Good Hunting.

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  • 1 year later...
Can someone definitively tell me what are the maximum size tires I can safely put on my 2006 RX400h? I drive my family in this car, so I don't want jeopardize our safety in any way.


You can run 255/55VR18, 245/55VR18, or 255/50VR18 tires on your OEM wheels without any safety issues. Be sure the load rating is at least 99 and do not drop down to lower speed ratings like H. Some minor rubbing in hard cornering may be found for the larger of those, but if you are doing that with your family in the car... then you really aren't that concerned about safety are you? :ph34r:

Your safest bet if you don't want to take any risk is to just get the the OEM tire size. The Bridgestone Dueler H/L Alenza is your tire of choice for this size.

CAUTION: I would avoid wheel spacers as they change the way loads are transfered from the wheel to the hub. These OEM wheels are desinged to fit over a feature on the hub so that the wheel bolts are only in tension (no shear loading). If you install spacers, this function is lost and the wheel bolts now have to carry tension AND shear loads. They were not designed for this. This is a REAL safety issue.

To move to wider tires without any clearance issues or spacers you will need to get different rims that have a greater offset from the hub. Just be certain thay engage the shear feature on the hub in the same way the OEM wheels do, or you will be placing the botls in shear again.

Good Hunting.


Sounds like you know what you are talking about! After much research, I am planning on going with the Michelin Latitude HP 255/55-18 104H, which is what comes standard on the Acura MDX. It has the closest overall diameter of any 255/55-18 tire so it makes it closer to the stock OD. I never go over 75MPH so I didn't think the H mattered and the MDX seems like a very similar SUV.
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Can someone definitively tell me what are the maximum size tires I can safely put on my 2006 RX400h? I drive my family in this car, so I don't want jeopardize our safety in any way.

You can run 255/55VR18, 245/55VR18, or 255/50VR18 tires on your OEM wheels without any safety issues. Be sure the load rating is at least 99 and do not drop down to lower speed ratings like H. Some minor rubbing in hard cornering may be found for the larger of those, but if you are doing that with your family in the car... then you really aren't that concerned about safety are you? :ph34r:

Your safest bet if you don't want to take any risk is to just get the the OEM tire size. The Bridgestone Dueler H/L Alenza is your tire of choice for this size.

CAUTION: I would avoid wheel spacers as they change the way loads are transfered from the wheel to the hub. These OEM wheels are desinged to fit over a feature on the hub so that the wheel bolts are only in tension (no shear loading). If you install spacers, this function is lost and the wheel bolts now have to carry tension AND shear loads. They were not designed for this. This is a REAL safety issue.

To move to wider tires without any clearance issues or spacers you will need to get different rims that have a greater offset from the hub. Just be certain thay engage the shear feature on the hub in the same way the OEM wheels do, or you will be placing the botls in shear again.

Good Hunting.

Sounds like you know what you are talking about! After much research, I am planning on going with the Michelin Latitude HP 255/55-18 104H, which is what comes standard on the Acura MDX. It has the closest overall diameter of any 255/55-18 tire so it makes it closer to the stock OD. I never go over 75MPH so I didn't think the H mattered and the MDX seems like a very similar SUV.

Go here and plug in your sizes and you'll see that with the OEM 235/55-18, the diameter difference with the 255/55-18 is not recommended. 255/50-18 is a better fit.

http://www.1010tires.com/tiresizecalculator.asp

Tire_Comps.pdf

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I am planning on going with the Michelin Latitude HP 255/55-18 104H, which is what comes standard on the Acura MDX. It has the closest overall diameter of any 255/55-18 tire so it makes it closer to the stock OD. I never go over 75MPH so I didn't think the H mattered and the MDX seems like a very similar SUV.

While the size is acceptable (within 3% of the OEM diameter), the speed rating of H is not.

It is unsafe to install a lower speed rated tire or a lower load rated tire than what is provided by the OEM.

Its not about how fast you drive but how the tire performs at speed and temperature. The only time it is acceptable to go with a lower speed rating is on on snow tires (and it goes without saying that you dont drive them in the summer).

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Okay, great. Thanks for the quick response.

Here is my concern: If I go with the 255/50-18 I will gain more miles since the tires are smaller, spin at higher REVS than the OEM and then I will be out of warranty sooner.

The 255/55-18 for the ACURA has a higher load rating on the Latitude HP (104) than the Lexus OEM at 99. There are lots of Acura's here in AZ with those tires and it weighs 4451 lbs. compared to the RX 400h at 4190 lbs., so it seemed to make sense. The Latitude HP for the Mercedes is a 105V tire but is also 10.6 wide instead of 10.4 and also has a OD of 29.2 instead of 29.1. There is a Latitude HP for the Porche that is 109V and 29 OD and 10.43 wide. Soooo many choices!!!

I did see that RXRey has had the TOYO Proxes for over 14 months with no problem but felt the Michelin might be the best quality. What is another $200 when you spend over $45K on the vehicle, especially on something that will last for about 3 years.

Your thoughts?

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Okay, great. Thanks for the quick response.

Here is my concern: If I go with the 255/50-18 I will gain more miles since the tires are smaller, spin at higher REVS than the OEM and then I will be out of warranty sooner.

The 255/55-18 for the ACURA has a higher load rating on the Latitude HP (104) than the Lexus OEM at 99. There are lots of Acura's here in AZ with those tires and it weighs 4451 lbs. compared to the RX 400h at 4190 lbs., so it seemed to make sense. The Latitude HP for the Mercedes is a 105V tire but is also 10.6 wide instead of 10.4 and also has a OD of 29.2 instead of 29.1. There is a Latitude HP for the Porche that is 109V and 29 OD and 10.43 wide. Soooo many choices!!!

I did see that RXRey has had the TOYO Proxes for over 14 months with no problem but felt the Michelin might be the best quality. What is another $200 when you spend over $45K on the vehicle, especially on something that will last for about 3 years.

Your thoughts?

Do the math, the 255/50/18's are only .51% undersize. thats only 102 miles extra in 20000 miles or 255 miles extra on 50,000 miles, not really a big difference in my opinion. Many dealers will give you a few miles over the max anyway.

:chairshot:

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Okay...I hate math. So now I know the smaller size doesn't impact warranty, which is great. Thank you!

I was also going to the 255/55-18 for a little more rim protection from curbs and little fuller wheel well. I did this on an RX 300 and it improved the look. I guess my only question for Skyfish or anyone else that really knows tires is the question about the Acura MDX 104H tire making that big a difference over a V tire. The Porche V tire is very close to the same size so that could work...Tire Size: 255/55R18/XL (MSPN: 26669). Just more money, what is another $100 when you ride on them for 40+K right (hopefully)! The Acura MDX just seems like such a similar car to the 400h than the Porche does.

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I guess my only question for Skyfish or anyone else that really knows tires is the question about the Acura MDX 104H tire making that big a difference over a V tire.

There probably is a relationship between load rating and speed rating that would allow you to trade speed for load, but I don't know what that formula is.

The point is that no reputable tire center will install lower speed rated tires then what is called for (unless thier snow tires). Pay the extra for the V rating.

I have the Goodyear Eagle Response Edge 255/55 running on my RX and couldn't be happier about their performance over the last 20K.

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Thanks. I appreciate the input and will go with the V.

The Goodyear costs a lot less...$198 vs. $268 each for the Latitude HP. They have nearly the same specs and the Goodyear also has a 50K warranty. Decision is getting easier.

How quiet are the Goodyear tires and have you noticed any drop in MPG?

I am getting about 28 MPG with mixed driving.

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Okay...I hate math. So now I know the smaller size doesn't impact warranty, which is great. Thank you!

I was also going to the 255/55-18 for a little more rim protection from curbs and little fuller wheel well. I did this on an RX 300 and it improved the look. I guess my only question for Skyfish or anyone else that really knows tires is the question about the Acura MDX 104H tire making that big a difference over a V tire. The Porche V tire is very close to the same size so that could work...Tire Size: 255/55R18/XL (MSPN: 26669). Just more money, what is another $100 when you ride on them for 40+K right (hopefully)! The Acura MDX just seems like such a similar car to the 400h than the Porche does.

You do not need a V rated tire the H rating is sufficient for any RX400h driver. What is more important is the load rating.

The speed rating V is good for a maximum speed of 149 MPH and the H is good for 130 MPH. I dont think the 130 MPH limit is an issue on the RX 400h.

The load rating is based on the ammount of air the tire holds along with construction, like the number of plies and steel belts.

I have a set of Bridgestone Alenza's that I took off of my Acura MDX just before I sold it, they are rated 109 H XL, they are extra load the same tire without the XL in that size is going to be 104 or 105 load rating. I sold my MDX when I bought the RX 400h and yet even though I had these tires with only around 400 miles on them I bought the correct size Bridgestone Alenza 235x55x18 for the RX 400h.

If you are interisted in the 255x55x18 Bridgestone Alenzas I still have them , contact me. I currently have them on a set of Acura MDX wheels but they could be unmounted. I am planning on posting the wheel and tire set on ebay this spring.

Look on the tire rack as it explains load and speed ratings in more detail.

Hear is a link to that info: http://www.tirerack.com/tires/tiretech/tec...?techid=35&

In other posts over the last year or so some have commented on having rubbing problems with the 255x55x18 in various tire makes. Look at the clearence in the rear wheel opening between your tire and the spring mounting plate. Also the rear fender liner. Its very close and depending on the individual fit at the factory some have experienced rubbing under certain driving conditions.

Personally I replaced the factory tires with Brigestone Alenza's 235x55x18 and am really satisfied with them. They are 100 V rated which is slightly better/ heavier load rating then the OEM Goodyears or Michelins. They feel like more tire than the OEM and performance wise according to customers that have purchased them from the tire rack, the perform much better in most areas then the OEM Goodyears and even the Michelins.

I myself, dont believe in oversizing on the diameter, and on a car like the RX 400h with close clearences I am not even big on increasing the section/tread width.

Anyhow good luck with whatever you pick.

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You do not need a V rated tire the H rating is sufficient for any RX400h driver. What is more important is the load rating.

Its not a good idea to put too much emphasis on the literal definitions for the speed ratings. They are only benchmarks for comparison. When you see 130mph, and think to yourself, "I would never drive an RX that fast, and neither should anyone else"... you are missing the point.

It's a capability, a safety factor, a margin for error, a tolerance. It's something I would rather have than not. Re-read the laboratory conditions for how they determine those speeds and all of the caveats they put around those ratings on the tirerack link you sited. And think again about the real world conditions they have to operate in.

Back on the size issue (size does matter ;)... Most any 255/55R18 tire will fit. Watch out for greater than 29" dia or 10.4" width. This size will give you a more stable ride, but will cost you 1-3mpg in fuel economy. The Michelin have the best MPG performance, but they handle like you're driving thru mud compared to my tires or the Toyos, or for that matter even the Bridgestone Alanza's which are a nice tire (and come in the OEM size). It's always something. Everything is a compromise.

The Toyos that some have tried fit a little better than the GYResponse Edge because they are 1/10th of an inch less wide in the wheel well and that may be enough to avoid the minor rubbing that I experienced when the GY were first fitted onto the car (or maybe he has never pushed his RX to its limits like I have B) ).

But now, after the first rotation I no longer have the rubbing (and I'm a bit more sedate in my driving... but I bet I could still get there. Come for a ride, let's find out :o )

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You do not need a V rated tire the H rating is sufficient for any RX400h driver. What is more important is the load rating.

Its not a good idea to put too much emphasis on the literal definitions for the speed ratings. They are only benchmarks for comparison. When you see 130mph, and think to yourself, "I would never drive an RX that fast, and neither should anyone else"... you are missing the point.

It's a capability, a safety factor, a margin for error, a tolerance. It's something I would rather have than not. Re-read the laboratory conditions for how they determine those speeds and all of the caveats they put around those ratings on the tirerack link you sited. And think again about the real world conditions they have to operate in.

find out :o )

Actually I am not missing the point but I think you are. I am a Mechanical Engineer and I understand tolerances, safety factors , testing methods and all that you mentioned!

I say again you dont need a V rated tire on the RX400h, and I again say the load rating is more much more important than the speed rating unless you plan on driving your RX400h on the autobahn at 130 MPH for long stretches of time.

Oh did you know that the OEM 17" tire that comes standard on the RX400h is a 101 S rated and if you didn't read carefully that is rated at 112 MPH. So in light of that fact I am 100% positive in knowing that the V rating on the 18" OEM's is not out of necessity.

One more point, the RX400h is electronically limited to a top speed of 112 MPH. hence the standard equipment tire rated at 101S.

How many SUV makes and models do you know of that have a top speed capability equal to the RX (112 MPH) and come with V rated tires.

Lets take the 07-08 Acura MDX for example since it uses the Michelin Latitude, a tire which comes in several different speed and load combinations. For example the Acura is fitted OEM with the 255x55x18 104H when they could have used the the same size 105V that is used on Mercedes or even the 109V XL which is used on the Porsche Cayenne. Heck even BMW does not use the V rated option they fit theirs with a 109H.

Now I know the RX 400h is a peppy SUV but it does not compare to the Porsche or BMW when it comes to being capable of being driven at high speeds and in agressive driving situations.

:chairshot: :cheers:

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You do not need a V rated tire the H rating is sufficient for any RX400h driver. What is more important is the load rating.

Its not a good idea to put too much emphasis on the literal definitions for the speed ratings.

It's a capability, a safety factor, a margin for error, a tolerance. It's something I would rather have than not.

Actually I am not missing the point but I think you are. I am a Mechanical Engineer and I understand tolerances, safety factors , testing methods and all that you mentioned!

I say again you dont need a V rated tire on the RX400h, and I again say the load rating is more much more important than the speed rating unless you plan on driving your RX400h on the autobahn at 130 MPH for long stretches of time.

You can say whatever you like. Its a free country. I'm only providing caution for the non-engineers out there who are not prone to second guessing another engineers work. I'm here to say that maybe, just maybe it would be better err on the side of safety. That perhaps you don't have the whole picture, and that there may be factors that you have not acounted for in your "analysis".

Here is a quote from the testing method on writeup on tirerack...

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.

It does not describe any evaluaton of the tire after the conditions have been met other than the implied condition that it has not been destroyed. They don't repeat the test day in and day out like we do in our cars over the years it take to wear out a set of tires. And they are not doing it on a real road with glass and rocks and nails and pot holes and and and....

You make your own choice. But allow other to have a different take.

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Following your reasoning skyfish, I take it that you only buy the highest speed-rated tires available?

Only ZR tires for the skyfish family? That would provide the highest possible margin of safety, right?

I understand your point, but speed rating is only one of many tire specifications and depending on application, speed rating may have very little or possibly no value whatsoever.

For example, when buying tires for my Rubicon, I have no idea what the speed rating is. I don't care, because even if dropped freefall out of a plane, the Jeep will not go over 90MPH due to the inherent horrible aerodynamics. I care about load rating, tread depth, tread pattern and wear rating(not necessarily a high treadwear, I need "soft and pliable", not "wear like iron").

When I choose a tire for my RX400, I will choose in this order: Size (need to go with stock fitment), Type of tire (probably a snow tire and probably a light-truck tire in my case), load range (this is a relatively heavy vehicle, hence the light-truck tire), treadwear (I need it to last), speed rating (at least S rated, I do not drive over 100mph, only rarely over 90).

Your point is well taken skyfish if you were choosing tires for your F-sport that you take to the track. Not really relevant for the wife's daily driver.

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Actually I am not missing the point but I think you are. I am a Mechanical Engineer and I understand tolerances, safety factors , testing methods and all that you mentioned!

I say again you dont need a V rated tire on the RX400h, and I again say the load rating is more much more important than the speed rating unless you plan on driving your RX400h on the autobahn at 130 MPH for long stretches of time.

Oh did you know that the OEM 17" tire that comes standard on the RX400h is a 101 S rated and if you didn't read carefully that is rated at 112 MPH. So in light of that fact I am 100% positive in knowing that the V rating on the 18" OEM's is not out of necessity.

One more point, the RX400h is electronically limited to a top speed of 112 MPH. hence the standard equipment tire rated at 101S.

How many SUV makes and models do you know of that have a top speed capability equal to the RX (112 MPH) and come with V rated tires.

Lets take the 07-08 Acura MDX for example since it uses the Michelin Latitude, a tire which comes in several different speed and load combinations. For example the Acura is fitted OEM with the 255x55x18 104H when they could have used the the same size 105V that is used on Mercedes or even the 109V XL which is used on the Porsche Cayenne. Heck even BMW does not use the V rated option they fit theirs with a 109H.

Now I know the RX 400h is a peppy SUV but it does not compare to the Porsche or BMW when it comes to being capable of being driven at high speeds and in agressive driving situations.

I guess I didn't know that mechanical Engineering would encompas all there is to know about rubber chemestry ... and how those varrying chemestires react under different loads and temperatures and durations and manufacturing techniques when mingled with other materials, or how the extra torque created by dual electric motors (as the AWD has), and extra battery weight has to be taken into account, comparred to the porsche/bmw / lighter suv's. My crazy thinking is that since a tire &/or auto manufacturer has express and implied warranties, that those kinds of responsibilities would carry more weight, than say if some shloemoe like me posts a thread that contradicts. I mean after all, if I'm wrong, buyer beware, right? Heck, even the tire companies don't know everything ... or am I the only one who remembers the SUV roll-over fatalities atributed to Firestone tires.

http://www.detnews.com/specialreports/2002...restonetire.jpg

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Following your reasoning skyfish, I take it that you only buy the highest speed-rated tires available?

Only ZR tires for the skyfish family? That would provide the highest possible margin of safety, right?

Classic straw man agruement...

There are a number of important factors in choosing new tires for your car. The most influential is probably price and warranty over anything else. The vast majority of buyers will choose tires that match exactly the OEM specs.

For some (like me), choosing a larger tire is justified to gain a performance edge (at the expense of fuel economy). But in doing so we are conscious not to sacrifice on any of the other key parameters.

I agree the most important factor that should NEVER be compromised is the load rating. That is straight forward enough that hardly anyone would argue against it, although there may be some.

I would also agree that the temperature rating should NEVER be compromised. Some could argue that they never drive in hot desert environs, and therefore do not need to match the temperature rating of the OEM tire. I would never do that for the same reasons I've articulated here on the speed issue.

My personal choice, obviously, is not to compromise on the speed rating for those same reasons. I'm not qualified to re-engineer these key parameters in the downward direction that has been proposed. One especially has to be careful of these choices when they are motivated by price. Rationalization is a powerful tool of the mind.

Certainly people are free to make their own choices. I just hope they have made their choices clear to the family and passengers that also use that vehicle so they can make their own choices.

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Not a straw man argument, not at all.

You seem to find fault with someone choosing a tire rated for the max speed a vehicle can possibly attain.

Apparently you advocate choosing an arbitrary safety margin of say 130mph on tires for a vehicle that is physically limited to 112, and when called on the arbitrary margin, you get testy about it.

And while we are talking logical fallacies, lets call your argument what it is: a false premise. You believe that the tire makers are exaggerating their ratings and that their ratings 1) cannot be believed and 2) have no margin of safety built-in.

This puts you in the odd position of arbitrarily determining which ratings are to believed and which are not while I believe that a posted rating on a major-manufacturer's tire can be believed. The tire makers have a vested interest in being honest and truthful in their ratings and test their products to these standards.

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Thank you everyone for points of view. Choosing a tire that you will drive for about 3 years is a very important issue.

I have researched this very carefully. From all the online research I have done, the Michelin Latitude HP 255/55-18 seems to be the highest quality tire that fits a 2006-2009 RX 400H. I did see negative issues with tires that fit the RX400 from Toyo, Goodyear and others and could find only positive reviews on Michelin. Discount Tire also said they have had the best satisfaction level from the Latitude HP.

There are several 255/55-18 Latitude HP tires from Michelin available. I narrowed it down to two that have the closest size specifications to the stock 235/55-18 (28.3 inches in overall diameter). I thought it would be best to get an answer directly from Michelin.

My Question to Michelin: The performance package comes with the 235/55-18, which is a V with a 99 load. As I mentioned in the last email, the Latitude HP made for the Acura MDX has an even higher load of 104. The Acura H is rated to 130 where the stock RX400 17 inch S is only to 112. So, based on that info...do you think the Acura 03727 (255/55-18, 104H) tire is better for the Lexus RX400h or is the Porsche 26669 (255/55-18, 109V) is better? It seems to me that Porsche tire might be overkill but you guys are the tire experts so I am counting on your guidance.

Michelin Answer: Assuming there are no clearance issues within the wheel wells, the Latitude Tour HP in size 255/55R18 104H (29.1 inches tall and 10.3 inches wide - Part Number 03727) or 109V (29 inches tall and 10.43 inches wide - Part Number 26669) may be an option. The Latitude Tour HP in size P255/55R18 104H would provide a comfortable ride and high performance handling. If you are looking for higher performance handling, then you may want to consider the Porsche version in size 255/55R18 109V XL. This tread design should provide even tighter responsiveness and steering wheel control, not to mention an even greater load carrying capacity. Of the two designs, the H-rated version may tend to last longer in regard to tread life.

I am going with the 104H and should have them on this week. I'll post before and after shots too.

Don

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Sounds like an informed, researched and satisfactory solution, Don.

I think any performance gains from the higher performing tires would be lost due to our heavy and squishy platform. The tires may indeed stick and be more responsive, but if the body rolls and lumbers over the tires...that would be a waste of capability. We have comfortable cars: Cayenne and X5 performance SAV's we are not.

Good Luck on your new shoes!

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Actually I am not missing the point but I think you are. I am a Mechanical Engineer and I understand tolerances, safety factors , testing methods and all that you mentioned!

I say again you dont need a V rated tire on the RX400h, and I again say the load rating is more much more important than the speed rating unless you plan on driving your RX400h on the autobahn at 130 MPH for long stretches of time.

Oh did you know that the OEM 17" tire that comes standard on the RX400h is a 101 S rated and if you didn't read carefully that is rated at 112 MPH. So in light of that fact I am 100% positive in knowing that the V rating on the 18" OEM's is not out of necessity.

One more point, the RX400h is electronically limited to a top speed of 112 MPH. hence the standard equipment tire rated at 101S.

How many SUV makes and models do you know of that have a top speed capability equal to the RX (112 MPH) and come with V rated tires.

Lets take the 07-08 Acura MDX for example since it uses the Michelin Latitude, a tire which comes in several different speed and load combinations. For example the Acura is fitted OEM with the 255x55x18 104H when they could have used the the same size 105V that is used on Mercedes or even the 109V XL which is used on the Porsche Cayenne. Heck even BMW does not use the V rated option they fit theirs with a 109H.

Now I know the RX 400h is a peppy SUV but it does not compare to the Porsche or BMW when it comes to being capable of being driven at high speeds and in agressive driving situations.

I guess I didn't know that mechanical Engineering would encompas all there is to know about rubber chemestry ... and how those varrying chemestires react under different loads and temperatures and durations and manufacturing techniques when mingled with other materials, or how the extra torque created by dual electric motors (as the AWD has), and extra battery weight has to be taken into account, comparred to the porsche/bmw / lighter suv's. My crazy thinking is that since a tire &/or auto manufacturer has express and implied warranties, that those kinds of responsibilities would carry more weight, than say if some shloemoe like me posts a thread that contradicts. I mean after all, if I'm wrong, buyer beware, right? Heck, even the tire companies don't know everything ... or am I the only one who remembers the SUV roll-over fatalities atributed to Firestone tires.

http://www.detnews.com/specialreports/2002...restonetire.jpg

Well let me try to explain my point, but first let me say what we have been discussing are mechanical properties, not chemical. Load rating, speed rating, tread wear, temperature rating and traction ratings are all mechanical properties. Also let me say that you dont have to be a chemist in engineering to understand mechanical capabilities of materials, in fact in collage I had to take courses such as "strength of materials" which involved the analysis of different materials which included their mechanical properties. I also had to take several chemistry courses, but no I am not a chemist. I do however understand things such as how the durometer value of rubber varies with its ability to withstand deformation and resist tearing (tensil strength) and how it will wear depending on the ammount of silica is in the rubber and how long the carbon strands are for example.

My background, well, I worked for Monsanto for a short time after I graduated from collage, but left them to work for an I C engine and Jet gas turbine manufacturer for the rest of my career. I worked in Research and Development part of my career and then in mmanufacturing engineering and quality troubleshooting. While I am not claiming to be an expert on this or any other subject as this is just a talk forum to exchange ideas, experiences and preferences.

My reference to being a mechanical engineer was mainly however directed at my understanding of how the tires are tested mechanically in a lab, tolerances and safety factors vs the statement that skyfish400h made that I should reread how the speed rating is established in the lab. Well I hate to scare anyone, but many ratings on other products are done the same way. Specifications are established, testing techniques are usually applied in a lab for example under controlled and repeatable conditions. Running the tire up to a speed for a duration of time without failure establishes a parameter for comparision. Even if you did the test on the road under actual conditions just how long would you drive an actual vehicle at 149 MPH + to determine if a trie was capable of a Z rating, until it blows out. I guess skyfish400h would do that. Sorry I feel ornery.

Finally to answer your questions 1.) The OEM standard equipment 17" tire is rated at 101 S, so that establishes the load rating that the vehicle needs and that is my point. Lexus as timid as they are when it comes to product liability (ie not allowing us to access the navigation while moving) believes that the 101 load rating and S speed rating which is 112 MPH, is sufficient. remember the 18" wheels are optional and so Lexus decided to go for a higher rated performance tire to appeal to customers who want that, not that it is necessary, otherwise the standard tire would have had a V rating too.

2.) I already ansewered your question on chemestry, we have been discussing mechanical properties, including your traction motor torque coment which just results in more tire wear do to slippage of the tire. Just like on the drag strip more wheel slipage more abraded rubber.

What I did on my 400h, well personally I agree with BillyShaft, I consider all the same aspects he mentioned in his post in much the same order, size, load rating, wear number, speed rating, temperature rating, traction rating. I purchased the Bridgestone Alenza 235x55x18 100 V. Granted it is an all season tire but what I was looking for was the wet and snow traction without the need to purchase a dedicated snow.

I do understand what you were trying to get at with your chemestry comment, today tires are more complicated than they used to be, but again the chemestry like long strand carbon filamants, added silica, dual durometer/variable chemestry tread layers ect., are only elements that vary the mechanical performance properties of the tire/rubber.

Beyond that I agree its a personal choice and there are trade offs and a little common sense is needed when selecting your tires. So far we have not discussed tread design and how higher performance tires while they grip better usually dont wear as well and dont ride as nicely. Lastly our 400h's aren't comparable to either BMW or a Porsche Cayenne in the performance and suspension capabilities. Therefore some of these tire choices are over kill and potentially wasted money on the 400h.

Well I had better stop this is getting way too long.

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I just received this email from Michelin today regarding a question about the Latitude HP for the Porche - Part Number 26669 (Why are the revs per mile higher than the stock 235/55-18 yet the overall diameter is larger?):

You helped us discover a mistake that we had on our Web site pertaining to the Michelin Latitude Tour HP in size 255/55R18 109V XL (Part Number 26669). The overall diameter is actually 28.03 inches, not 29.1 like the P255/55R18 104H (Part Number 03727, rev per mile is 720). That is the reason the rev per mile is greater (742) on the Porche tire compared with stock Lexus size 235/55R18 (736).

If you really want a sporty RX400H that is probably the tire to go with. It is slightly lower than the stock tire and is the widest that will fit (10.43 wide).

I still plan on going with the 255/55R18 104H (Part Number 03727) because:

1. I prefer a little more tire protection from curbs and 1 inch could make the difference

2. I want the tires to last longer than the V rated Porche tire

3. The price is $240 compared to the Porche tire, which is over $300

4. I like the look of a fuller wheel well

5. I can deal with the speedo reading 67 when I am going 65

6. The 104H is probably overkill for the RX400H anyway

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Not a straw man argument, not at all.

You seem to find fault with someone choosing a tire rated for the max speed a vehicle can possibly attain.

Apparently you advocate choosing an arbitrary safety margin of say 130mph on tires for a vehicle that is physically limited to 112, and when called on the arbitrary margin, you get testy about it.

And while we are talking logical fallacies, lets call your argument what it is: a false premise. You believe that the tire makers are exaggerating their ratings and that their ratings 1) cannot be believed and 2) have no margin of safety built-in.

This puts you in the odd position of arbitrarily determining which ratings are to believed and which are not while I believe that a posted rating on a major-manufacturer's tire can be believed. The tire makers have a vested interest in being honest and truthful in their ratings and test their products to these standards.

Yes it was a strawman argument. I never advised getting better rated tires, only that one should not use a lower rating that what came with the car. The fact that the 17" tire has a different rating is apples and oranges. If I was in the market for a 17" tire, I would not get less than an S rated tire. Does that help clear things up?

Your perception of "testiness" is incorrect. I am only providing a point of view that seems to be missing from the discussion.

It's always amusing when someone tries to say what another believes. It's almost universally wrong. What I believe has been stated but I will try again.

Let me put it another way. If the tire makers felt that it was advisable to print "MAXIMUM SPEED 130 MPH" on the side of a tire, don't you think they would? They certainly do so for the pressure...

No, the reason they use the convoluted (intentionally confusing?) H, S, V, ZR rating system is likely because that forces the consumer to READ how those ratings are determined and to NOT take to the MPH ratings literally (this gets legal off their backs). There are too many differences between the lab conditions and the real world to make such a bold claim.

Your conclusion is based on a literal interpretation of the numerical rating on side of the tire. Extrapolating those values from the lab conditions to the real world... that is the fallacy.

Having said all that, everyone is free to make up there own mind. I feel I've defended this position long enough. It's time to move on.

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i run 255/55/18 Pirelli P-Zero Scorpion Ice/Snow tires on my stock wheels now, ABSOLUTELY love them, got them used on ebay and I couldn't be happier ,10k miles this winter and still going strong, ALOT stiffer than the stock michieln garbage, handle MUCH BETTER In the dry as well, not to mention the RX400h actually handles decently in the snow now, highly reccomend these tires, NO PROBLEM with fitiment either.

I run 255/45/20 Toyo Proxes 4 for the summer, im not sure if these are SUV tires but they are awesome for the rain/dry, i can corner much better but i actually like the Pirelli's much better..

looking to go 275/40/20 this summer... i'll fill you guys in if they fit or not, probably a matter of spacer size lol since i run a 5mm spacer on my 20" wheels :)

stock tires suck, get rid of them ASAP!@!!!!!!!!!!!!!!

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