Speedo Accuracy

[BigGlenn]

I have always had an inaccurate speedo in my 2007 LX470 (current odometer - 29k km's). Using my Garmin Nuvi 760 GPS, when the GPS shows 60kmh the speedo shows about 63kmh, when the GPS shows 100kmh the speedo shows 105kmh and when the GPS shows 110kmh (maximum speed in NSW) the speedo shows nearly 120kmh. My local Lexus dealers service centre says this is normal and "within specifications". You would think with todays electronic systems they could get a speedo to measure accurately? Or, am I just an OCD Engineer who expects Perfection (or at least the persuit of it?) <_<

I should note, my LX has the 17" Lexus wheels with Dunlop Grandtrek AT22 275/65R17 115T tyres.

[NEG]

I have always had an inaccurate speedo in my 2007 LX470 (current odometer - 29k km's). Using my Garmin Nuvi 760 GPS, when the GPS shows 60kmh the speedo shows about 63kmh, when the GPS shows 100kmh the speedo shows 105kmh and when the GPS shows 110kmh (maximum speed in NSW) the speedo shows nearly 120kmh. My local Lexus dealers service centre says this is normal and "within specifications". You would think with todays electronic systems they could get a speedo to measure accurately? Or, am I just an OCD Engineer who expects Perfection (or at least the persuit of it?) <_<

I should note, my LX has the 17" Lexus wheels with Dunlop Grandtrek AT22 275/65R17 115T tyres.

It is the same on my Ls400, my Porsche and was the same on my VW - I think it is deliberate!

[BigGlenn]

I have always had an inaccurate speedo in my 2007 LX470 (current odometer - 29k km's). Using my Garmin Nuvi 760 GPS, when the GPS shows 60kmh the speedo shows about 63kmh, when the GPS shows 100kmh the speedo shows 105kmh and when the GPS shows 110kmh (maximum speed in NSW) the speedo shows nearly 120kmh. My local Lexus dealers service centre says this is normal and "within specifications". You would think with todays electronic systems they could get a speedo to measure accurately? Or, am I just an OCD Engineer who expects Perfection (or at least the persuit of it?) <_<

I should note, my LX has the 17" Lexus wheels with Dunlop Grandtrek AT22 275/65R17 115T tyres.

It is the same on my Ls400, my Porsche and was the same on my VW - I think it is deliberate!

NannyState -> No Nav/Screen entry on the move and Speedo's that under-read your speed - we can't be trusted!

[strong1993]

I have always had an inaccurate speedo in my 2007 LX470 (current odometer - 29k km's). Using my Garmin Nuvi 760 GPS, when the GPS shows 60kmh the speedo shows about 63kmh, when the GPS shows 100kmh the speedo shows 105kmh and when the GPS shows 110kmh (maximum speed in NSW) the speedo shows nearly 120kmh. My local Lexus dealers service centre says this is normal and "within specifications". You would think with todays electronic systems they could get a speedo to measure accurately? Or, am I just an OCD Engineer who expects Perfection (or at least the persuit of it?) <_<

I should note, my LX has the 17" Lexus wheels with Dunlop Grandtrek AT22 275/65R17 115T tyres.

I am also OCD about these things (or CDO if you prefer). I've checked my speedo with GPS many times and it's within 99% to 100% accurate at the speeds I've tested (no higher than 85...yet), if speedo indicates 60 MPH I'm going 60.0 to 60.5 MPH.

[BigGlenn]

I have always had an inaccurate speedo in my 2007 LX470 (current odometer - 29k km's). Using my Garmin Nuvi 760 GPS, when the GPS shows 60kmh the speedo shows about 63kmh, when the GPS shows 100kmh the speedo shows 105kmh and when the GPS shows 110kmh (maximum speed in NSW) the speedo shows nearly 120kmh. My local Lexus dealers service centre says this is normal and "within specifications". You would think with todays electronic systems they could get a speedo to measure accurately? Or, am I just an OCD Engineer who expects Perfection (or at least the persuit of it?) <_<

I should note, my LX has the 17" Lexus wheels with Dunlop Grandtrek AT22 275/65R17 115T tyres.

I am also OCD about these things (or CDO if you prefer). I've checked my speedo with GPS many times and it's within 99% to 100% accurate at the speeds I've tested (no higher than 85...yet), if speedo indicates 60 MPH I'm going 60.0 to 60.5 MPH.

strong1993, what wheel/tyre combo do you have, I suspect this is my problem.

[cduluk]

The dealer should be able to adjust this.

I know when you switch wheel size, the dealer can adjust the speedometer computer to compensate for the difference in wheel circumference.

Or is the problem more consistency?

[BigGlenn]

The dealer should be able to adjust this.

I know when you switch wheel size, the dealer can adjust the speedometer computer to compensate for the difference in wheel circumference.

Or is the problem more consistency?

Actually, come to think of it, I did notice a calibration option for new tyres - I might give that a go and see what it does. <- just looked at the manual, it looks like this is just for the Nav function - I assume when you loose GPS signal like in a tunnel it's used to show your position correctly.

The problem is consitent inaccuracy - for example when the speedo says 100kmh the GPS (external Garmin Nuvi 760) says 95kmh and when the speedo says 105kmh the GPS says 100kmh.

I was thinking I should get new tyres with a 5% larger circumfrence - going from the 275/65R17 tyres to 285/70R17 - I think they will fit but it seems a waste if the dealer could adjust the calibration (did request that my dealer did this but they claimed it was within spec) - I'd prefer Lexus trying to rectify a design bug (?) rather than me spending $2k on new tyres (especially when my existing ones are only 30k km's old)

[BigGlenn]

Any thoughts on changing my tyres to fix this problem?

[pauljcl]

Any thoughts on changing my tyres to fix this problem?

On my 09RX, the speedometer indicates +3 mph vs. actual, consistently over the range of 50 to 100 mph. This is with new tires pressured to 32-33psi (instead of the 30 psi recommended). I know it, so I live with it. I did not know the dealers could tune the speedometer, I always thought it was 'a big deal' involving changing some computer or ECU. You may be interested in the following site:

http://www.miata.net/garage/tirecalc.html

which allows you to easily calculate the effect of a different tire size.

Speedometer error - when speedometer overstates - obviously increases with tire wear (though I haven't calculated by how much - tread depth when new is often 10/32") and decreases with higher psi. It is unlikely that you would consistently get a correct speedometer reading for an extended period of time....

Let me know....

[hoser]

Search for "Speedometer calibrator" and you'll find several choices from $80-200. They are devices that modify the electronic signal from the transmission to recalibrate the speedometer (and ECU). Very popular in the off-road world.

[cduluk]

As long as the error in speed is liner, it can be fixed. It should be a simple "plug into computer and press a few buttons" thing for the dealer to do...

[hoser]

The OP's vehicle IS within spec/tolerance and yes, it would be difficult even with today's technology to get the speedo dead accurate without using a GPS. Unless your tires always have the same pressure which means its always the same temperature, you travel at only one speed and the rubber on your tires never wear down, it is nearly impossible. Tires even change diameter at different speeds.

Yes, it SHOULD be as simple "plug into computer and press a few buttons" thing for the dealer to do but unfortunately it is not for the LX470. If anybody can prove otherwise, I'd surely be grateful to know.

[BigGlenn]

As long as the error in speed is liner, it can be fixed. It should be a simple "plug into computer and press a few buttons" thing for the dealer to do...

Yes, the error seems to be linear. Whilst driving today when the speedo indicated 100kmh the GPS showed 95kmh, when the GPS showed 100kmh the speedo showed 105kmh. So it looks like a 5% over read problem by the speedo - therefore, if I had wheels with a 5% larger diameter (285/70 as opposed to 276/65) it should fix the problem.... now where did I put that $1,000 for new tyres? :whistles:

[BigGlenn]

The OP's vehicle IS within spec/tolerance and yes, it would be difficult even with today's technology to get the speedo dead accurate without using a GPS. Unless your tires always have the same pressure which means its always the same temperature, you travel at only one speed and the rubber on your tires never wear down, it is nearly impossible. Tires even change diameter at different speeds.

Yes, it SHOULD be as simple "plug into computer and press a few buttons" thing for the dealer to do but unfortunately it is not for the LX470. If anybody can prove otherwise, I'd surely be grateful to know.

Agree hoser, my dealer did not offer a solution other than stating that the speedo was within spec - the Australian road rules apparently ask for a tolerance of plus/minus 10%!! "She'll be right MATE!!"

[hoser]

Glenn, this is what you need.

http://www.marks4wd.com/products/tyre_matc...correction.html

I actually don't need correction on mine. I increased the diameter of my tires by 4" but also changed my gearing from 4.3 to 4.88 and the speedometer is dead on according to my GPS.

[smooth1]

Here try this, take 3 different GPS units like a Nuvi, a Garmin and some other one. Do 25 mph and see if any of them are correct or match, including your speedo, then do 60 and look again. GPS measures your speed by calculating distance traveled since the last time the signal was recieved. So in some cases your actually getting an "average" speed over the last xxx feet or whatever distance unit it measures in. GPS does get more accurate at higher speeds though. You also have to take into consideration how many corrolators and correction programs are used in each unit. And also the quality of the signal to and from the satilite. Most all states have a 5% "grey"area just for this issue. As tires wear down they get smaller which changes the accuracy of your cars speedo, but it would be in your favor as your speedo would read faster than you are actually going.

[BigGlenn]

Glenn, this is what you need.

http://www.marks4wd.com/products/tyre_matc...correction.html

I actually don't need correction on mine. I increased the diameter of my tires by 4" but also changed my gearing from 4.3 to 4.88 and the speedometer is dead on according to my GPS.

hoser, what tyres did you get and how did you change your ratio?

[hoser]

I have 35" Toyo MT tires. When I say change gear ratio, I'm talking about the front and rear differentials. Any good differential shop can do it. Not cheap though.

[BigGlenn]

Here try this, take 3 different GPS units like a Nuvi, a Garmin and some other one. Do 25 mph and see if any of them are correct or match, including your speedo, then do 60 and look again. GPS measures your speed by calculating distance traveled since the last time the signal was recieved. So in some cases your actually getting an "average" speed over the last xxx feet or whatever distance unit it measures in. GPS does get more accurate at higher speeds though. You also have to take into consideration how many corrolators and correction programs are used in each unit. And also the quality of the signal to and from the satilite. Most all states have a 5% "grey"area just for this issue. As tires wear down they get smaller which changes the accuracy of your cars speedo, but it would be in your favor as your speedo would read faster than you are actually going.

smooth1, agree 100% with what you say - the issue I have is with a AU$100k+ vehicle made in April 2007 by one of the worlds best motor manufacturers they can't get the speedo to be accurate. I can apply the +5kmh re-calibration (checked with two simultaneous GPS' versus my speedo) by my brain but sheesh!