Is gas mileage based on engine RPM or the throttle body?

I know in most cases they go hand in hand (WOT=high RPM's) but think about this: If im going 45 mph in 5th gear in SS mode and i floor it, the throttle is wide open, but the RPMs are still under 3000.

My question is: will a WOT, even under 3000 RPMs give me poor gas mileage? Or is gas mileage based on RPMs only? Such as no matter what the position of the throttle, the only factor that affects gas mileage is RPMs.

I hope I make sense.

Both. Throttle position is probably more significant than rpm, but running an engine in too low of a gear on a nearly closed throttle increases pumping losses. A BMW study years ago (which lead to the 1980's E series engines) showed WOT was efficient for acceleration asuming the car was put in top gear as soon as the desired speed was achieved, even if the engine was redlined getting there. BMW achived maximum city fuel economy by using WOT, shifting at 2K rpm, and putting the car in top gear ASAP. Having tried this, it looks a bit weird to an outsider as the car sort of hops foward at each shift.

Engine temperature, outside temperature, fuel specific heat (varies winter to summer and with or without oxygenates), fuel octane rating, head winds, tire pressures, tire rolling resistance, dirt on car body, roof racks, etc. all make a difference as well. A driver who is sensitive to engine load and rpm can best Hondas EPA estimates, even though averaging higher than posted speeds (from experience).

pettydw does a nice job of explaining the numerous variables associated with fuel consumption. But I would strongly disagree with BMW's tests of using WOT to obtain the desired cruising speed as being condusive to top fuel economy.

I'm not saying the tests were not conducted, nor that the results are not as reported. I'm just saying that this would be highly unusual and would seem to violate all conventional wisdom to the contrary. Perhaps their engines were tuned specifically to flurish under this kind of use.

That said, as pointed out, there are SO many variables affecting the ability of an engine as installed in a vehicle to deliver a given fuel consumption. Certainly engine speed (RPMs) and load are right up there. I wouldn't worry about it too much unless your TL is delivering uncommonly low fuel economy. I average between 26 and 27 MPG in the summer months in city and suburban driving, no special mileage techniques employeed, and with the A/C used maybe 25 to 50% of the time. In the winter, my mileage drops to the 24 to 25 MPG range under the same conditions (no A/C, of course). On the road in the summer at speeds of 60 to 62 MPH and with the A/C on constantly, I get 32 MPG.

Oh, my TL is an '04 with a manual transmission.

I'd be willing to bet you could figure out optimum RPM shift points during slow acceleration if you watch how an A/T shifts (except for the 4-5 shift, it comes too late IMO) -- of course you'd need an A/T TL, and you'd have to adjust for your 6-speed.

Plausible theory... but I think you're forgetting that the ECU learns your driving habits and shifts accordingly. And during slow accel, you may not even feel the shifts (I don't think the screen displays the selected gear unless your in SS mode, or does it?)

BMW's tests got the car into the highest gear for the posted speed ASAP, minimzing time at WOT. When idling or at low throttle angles, your car's engine is working against a substantial vacuum (25" Hg typical). Diesels kick b_tt in the city cycle because they do not have an air restricting throttle plate. BMW's savings by reving to the redline was only a couple percent. The savings by shifting at 2K rpm was on the order of 20%. BMW's tests were well documented by Car and Driver and other auto magazines in the mid 1980's.

Automatic transmission owners need not concern themselves, as the automakers have preprogrammed the cars for maximum economy and emissions, regardless of your driving style. Any auto-learn routine will still try to optimize for these key parameters at the expense of anything else. Manual transmissions are on there way out because automakers cannot preprogram how individuals will drive and still meet the EPA requirements. Be thankful for the choices you have now.

pettydw, thank you very much for the write up. I do however, have a question for you. Are Diesel fuels a source of polution? I do see pitch black smokes coming out of the exhaust from Diesel engines....

Just wondering....

thanks

Diesel engines pollute far more than today's gasoline engines. This doesn't mean that improvements could narrow the gap. But keep in mind, when you say diesel, you are including trucks and machines of all kinds with diesel engines which easily exceed the numbers of cars on the road with diesel engines.

Here's one for you. In 1996, the lowest polluting engines produced fewer contaminants running than a 1966 engine did when not running. And that was 10 model years ago.

I have some answer for the original question. Try this : reset your travel computer when you are cruising on highway speed, use SS mode and stay in 5th gear, FLoor it. You will see gas mileage going relaly BAD instantly.

High RPM eats more gas - i knew that
as well as opening of throttle regardless of RPM - I found out doing above

I've used that technique also since reading those articles about BMW's findings in the early 80's. Short shifting at 2K-2.5K RPM at WOT, much harder to do with my 6MT TL than other lower power 4 cylinder MT Honda/Acura's.


BMW also made alot of changes to the "eta" engine for the 1983 528e which replaced the previous gen 528i. They used light valve springs and the motor was tuned for high torque at low RPM, so it only redlined to 4750RPM (my brother had a 325 with the same motor). HP was not great but that was a very efficient 6 cylinder for it's time.

Thanks