Better Gas Mileage With Premium

[RX400h]

RX,

Your analysis of Lexus’s fuel octane recommendations in the O&M manual are not correct. Lexus says regular - 87 pump octane is acceptable and 91 pump octane is recommended. In the US pump octane is an average of ROM and MON. IE 87 pump octane = (91 ROM + 83 MON ) /2.

The proof is using the fuel. There is no knocking or reduced power as a result of using 87 octane.

Ron

Ron,

It is actually a bit ambiguous. The manual does not state that "pump" ratings are to be used, although this can easily be inferred by the nomenclature. The manual states "Octane rating of 87", but never mentions "MON" as an averaging factor. 87 pump octane does seem to work adequately during cooler ambient temperatures, but I would shy away from using it during the Summer months.

Dave

[bikeandsail]

RX,

Your analysis of Lexus’s fuel octane recommendations in the O&M manual are not correct. Lexus says regular - 87 pump octane is acceptable and 91 pump octane is recommended. In the US pump octane is an average of ROM and MON. IE 87 pump octane = (91 ROM + 83 MON ) /2.

The proof is using the fuel. There is no knocking or reduced power as a result of using 87 octane.

Ron

Ron,

It is actually a bit ambiguous. The manual does not state that "pump" ratings are to be used, although this can easily be inferred by the nomenclature. The manual states "Octane rating of 87", but never mentions "MON" as an averaging factor. 87 pump octane does seem to work adequately during cooler ambient temperatures, but I would shy away from using it during the Summer months.

Dave

Dave,

The 91 RON in parrenthis implicitly indicates that the 87 is pump octane. IE pump octane 87=91 RON - approximatley. There is no ambiguity to me. I know what knocking is and in 60,000 miles, maybe 10,000 towing a boat t u[ tp 80 mph I have never heard a knock or had a loss of power.

All my testing on premium has indicated no better and possible worse gas mileage contary to Lexus claims.

Ron

[Ramy]

I drove my 400 the first 1500 miles with premium only, averaged 24.7MPG on mix driving, mostly short hwy trips. The next 1500 miles or so I used regular and averaged 24.5MPG on the same type of driving, and no noticable performance difference. So my experience is that using premium will improve your MPG by about 1% but will cost you about 7% more. I see no reason to use premium.

Ramy

OK, for what it's worth... I have tried several tankfulls of 92 premium unleaded fuel, even tried a couple of 94 Super Unleaded Premium fuel... but, I get the BEST gas mileage using 87 Regular Unleaded Fuel.

I consistantly get 1.00 - 1.5 mpg better with Regular Unleaded, all things being equal. Most of my driving is 50% city and highway (during rush hour).

I average about 31 + or - 1mpg in the late spring, or early fall, and average about 33mpg + or - 1mpg in the hot summer months.

Everytime I ran with super unleaded, I could not attain the same mileage.

My Best ever recorded mileage with Regular Unleaded was 34+ mpg.

My Tire Pressure is about 37-38 psi with Good Year Eagle ResponsEdge.

I see no point in running Higher grades of fuel or spending more on the Octane, given the fact that it is NOT performance in terms of raw speed we are after but rather fuel economy. The HIGHer octane is designed for one thing, and that is to reduce detonation (usually occurs under heavy load). That being said, I do not see how higher octane can yield better fuel economy since we are not Loading up the engine.

Cheers,

MadloR

[RX400h]

RX,

Your analysis of Lexus’s fuel octane recommendations in the O&M manual are not correct. Lexus says regular - 87 pump octane is acceptable and 91 pump octane is recommended. In the US pump octane is an average of ROM and MON. IE 87 pump octane = (91 ROM + 83 MON ) /2.

The proof is using the fuel. There is no knocking or reduced power as a result of using 87 octane.

Ron

Ron,

It is actually a bit ambiguous. The manual does not state that "pump" ratings are to be used, although this can easily be inferred by the nomenclature. The manual states "Octane rating of 87", but never mentions "MON" as an averaging factor. 87 pump octane does seem to work adequately during cooler ambient temperatures, but I would shy away from using it during the Summer months.

Dave

Dave,

The 91 RON in parrenthis implicitly indicates that the 87 is pump octane. IE pump octane 87=91 RON - approximatley. There is no ambiguity to me. I know what knocking is and in 60,000 miles, maybe 10,000 towing a boat t u[ tp 80 mph I have never heard a knock or had a loss of power.

All my testing on premium has indicated no better and possible worse gas mileage contary to Lexus claims.

Ron

Ron,

I'm fairly certain that the Lexus ECU acts similarly to that in my Corvette: When intake air temps exceed a certain threshold or pinging is heard by knock sensors, the ECU retards timing to prevent (further) predetonation. With over 200 HP, you may not feel a 10 HP loss under most driving conditions, but I guarantee you that if you were to instrument your car to measure acceleration times, you'd see an increase in time to accelerate to 60 MPH or more if using regular fuel vs midgrade or premium.

Granted, this difference may not be worth the cost increase to most of us, but that doesn't mean that there is no difference at all.

Dave

[Gryphon]

Dave, that is exactly what the Lexus engineers told me on the early 90s about running regular in my LS400. They also pointed out that retarded timing can result in higher engine temperatures, which likely contributed to my numerous engine-related failures I experienced. The engineers were not ducking responsibility, by the way, I had an extended warranty which thankfully covered several thousand dollars in repairs.

However unlikely running regular gas may be to causing future problems with my RX400h, I am not going to experiment to save perhaps $100-200 per year.

Tom

[RX400h]

Tom,

Seeing that you live in Arizona, I'd say you are doing the right thing. The ambient temperature makes a big difference when it comes to the likelyhood of predetonation.

Dave

[NeoN]

We are getting less mileage with cooler temps (40s - 60s) - the engine runs more to keep at operating temp - premium has consistently added 1.8 to 2.6 miles per gallon - we have 24,000 miles on our RX400 and have averaged a true 24.3 mpg - we use middle grade as a compromise for fuel unless heading up into the mountains.

My wife drags our overall (15,500miles) mpg down to 26.0 using regular. Ours is AWD. Part of this is from Montana winter driving too. Mostly though it's So. Cal driving. When I was the primary / solo driver I could regularly get 30mpg ... using pulse driving I learned with the prius. My wife though, opts for the cheep gas. I'll see if I can get her to consistently use primium and see if that makes a difference.

what do you mean by "pulse" driving?

[crabman]

First post here guys. There is a little misinformation in this thread. Gas is a commodity. Trucks from all manner of brands fill up out of the same tanks at refineries and storage depots all over the states. The product is differentiated (one brand to another) only by the additive package which is added when the trucks fill up and is a very small quantity. The octane rating of gas tells you the speed with which the gas burns and its resistance to detonation. Gas of any octane has the same energy density, 45 megajoules per kilogram. This means that premium has no economy advantage. The benefit to running higher octanes is that it allows a higher compression ratio and/or more timing/advance. Any of these things or combinations of these things will bring an increase in power and efficiency. The downside is that as you increase your compression or add timing it makes it easier for the gas to pre-detonate. This is the knock. Modern computer controlled engines run on the 02 sensors most of the time to control the mixture and a fixed timing at various loads and rpms. During large throttle openings or WOT the car runs a fixed value for the mixture based on rpm and has two timing values it can run on. These timing values are a standard timing value set and a low octane table that it can revert to if it detects knock from the knock sensors. On some cars the lower timing value only serves to protect the engine in the event of a bad tank of gas. These cars will not gain any power during large or full throttle applications by using premium because the values were set to be well within safe tolerances using regular gas. On other cars manufactures have taken advantage of the ability of higher octanes to resist detonation by giving them more compression and or timing where they can safely run on premium without detonation but will not running regular. These cars can gain extra power by running premium but it is only seen at large throttle openings. Typically, cars are set to stay on the low octane table till the car is filled up again. Sometimes it must be filled up more than once. At this point it is back on the standard tables until knock is detected again. Engine longevity is not effected by octane unless the car experiences detonation. Your warranty cannot be voided by running regular unless the car requires it. This must be stated in the manual and warranty information. Very few cars do, for most it is recommended rather than required. To go back to the beginning there is no economy advantage to be had from running premium. It contains no more energy and your car is running on the 02 sensors which operate at the same burn efficiency with either grade.

[dadcat]

We are getting less mileage with cooler temps (40s - 60s) - the engine runs more to keep at operating temp - premium has consistently added 1.8 to 2.6 miles per gallon - we have 24,000 miles on our RX400 and have averaged a true 24.3 mpg - we use middle grade as a compromise for fuel unless heading up into the mountains.

My wife drags our overall (15,500miles) mpg down to 26.0 using regular. Ours is AWD. Part of this is from Montana winter driving too. Mostly though it's So. Cal driving. When I was the primary / solo driver I could regularly get 30mpg ... using pulse driving I learned with the prius. My wife though, opts for the cheep gas. I'll see if I can get her to consistently use primium and see if that makes a difference.

what do you mean by "pulse" driving?

Pulse and glide driving is where you accelerate past your desired speed then coast down to below your desired speed and repeat this technique to maximize your fuel ecomny. The following link has more info: http://www.metrompg.com/posts/pulse-and-glide.htm

[NeoN]

We are getting less mileage with cooler temps (40s - 60s) - the engine runs more to keep at operating temp - premium has consistently added 1.8 to 2.6 miles per gallon - we have 24,000 miles on our RX400 and have averaged a true 24.3 mpg - we use middle grade as a compromise for fuel unless heading up into the mountains.

My wife drags our overall (15,500miles) mpg down to 26.0 using regular. Ours is AWD. Part of this is from Montana winter driving too. Mostly though it's So. Cal driving. When I was the primary / solo driver I could regularly get 30mpg ... using pulse driving I learned with the prius. My wife though, opts for the cheep gas. I'll see if I can get her to consistently use primium and see if that makes a difference.

what do you mean by "pulse" driving?

Pulse and glide driving is where you accelerate past your desired speed then coast down to below your desired speed and repeat this technique to maximize your fuel ecomny. The following link has more info: http://www.metrompg.com/posts/pulse-and-glide.htm

Great article, too bad this doesn't work for my daily 3.5 mile commute.

Has anyone had any experience with pulse driving? Any tips or tricks for RX400h drivers?

[Gryphon]

Crabman. That is an excellent post. Thanks for sharing your knowledge.

Tom

[Jim Clark]

Agree. Excellent post by crabman. The way I read it, there's no economic benefit to premium and no real power benefit. What about engine health? Does premium help prevent carbon build-up? Are there ANY benefits to premium?

[crabman]

Bottom line is I dont know on the vehicles we are discussing here. My wife has purchased an RX and Im just now starting to research the vehicle. Many cars, typically running higher power outputs can benefit from premium because it keeps them off the low tables. Significant power is lost by pulling timing and you dont want that when the point is to have fun on a backroad romp. One thing to keep in mind is that in normal operation on the O2 sensors and fixed tables nothing is lost because the OEMs keep the timing conservative in those operation parameters. This is because we are not asking much from the engine so there is no reason to run a more aggressive timing. It is only at larger throttle openings that power is lost if you have been kicked to the low octane tables. As already stated there is no economy benefit. All commercially available gas has the same energy density and the 02 sensors will keep the mixture as close to perfect (14.7:1) as is possible in a given engine at the same efficiency with any octane. Engine longevity is only affected by octane ratings if the particular setup goes into detonation, this will significantly reduce engine life.

[RX400h]

I think the benefits of Techron in fuel are well established. I throw in a can at least once per year.