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I was reading a thread in a car forum where someone wrote:

The lower the RPM, the less fuel you use, the less strain you put on everything..

Another person then replied, stating:

Not true. If your pulling high gears and making the engine work more to go how you want your using more gas. As well as putting more strain on the car. By wanting more from the car when it is out of its power your forcing the injectors to open more thus burning more gas. Please don't mislead anyone if your not sure. However, waiting until redline to shift isn't ideal either. Shifting at 3/4 or so of the torque curve is optimal for gas.

Who is correct? Is it more gas efficient to to shift at lower RPMs rather than high RPM? Would something like this vary great from car to car, model to model, or would it be a general rule for all standard engines?

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While someone will probably provide a detailed answer, internal combustion engines have a certain range where they are most efficient. This is affected by many factors so it isn't a black and white answer. There is a specific rpm point where any given engine is most efficient. It isn't always at a lower or higher rpm point. –  Rig Oct 20 '11 at 22:54
    
Thanks Rig. Do you think I should tailor the question specifically to my car or leave it relatively broad as it is and ask another question specific to my car's engine? –  stoicfury Oct 20 '11 at 22:56
    
Might not hurt but I don't know that there is a specific answer to be had either. –  Rig Oct 20 '11 at 23:17

5 Answers 5

up vote 3 down vote accepted

I think you need to define your terms. Efficiency could mean at least four things in this context:

  1. Least fuel consumed per hour.
  2. Most mileage covered per unit of fuel.
  3. Most kinetic energy created per unit of fuel consumed.
  4. Most acceleration created per second.

These four choices often destructively interfere with each other. Hitting some of the high points of each of the above meanings:

  1. High gears usually mean lower revs meaning lower amounts of fuel injected on average. The squirts of fuel might change in size depending on throttle position but those squirts happen less often.
  2. Mileage per fuel units is a multivariate equation in which aerodynamic drag starts to dominate at higher highway speeds. Even so, if you are driving very slowly, you will also see lower mileage than the optimum as you just aren't covering much linear distance.
  3. Internal combustion engines almost always have a peak energy output point where they are best able to combine fuel and air and transform that into kinetic energy. This is usually the torque peak of the engine. Note: peak torque also means near peak fuel consumption per second. Also remember that an engine's torque peak has nothing to do with the current gear selection.
  4. Peak acceleration is likewise tied to the torque peak but also requires proper gear selection and involves aerodynamic and static drag factors. Best acceleration from a stop will use the lowest gear. At speed, low gears will not be a choice as you'd have to rev the engine past its physical limitations, which is one of the many reasons why your acceleration drops off as speed increases.

Note that all factors are more complicated than can be addressed in a single page. For example, my turbo standard transmission car injects even more fuel than you'd expect at its torque peak. The engine computer is using that extra fuel to cool the higher compression fuel-air mixture in order to avoid premature detonation. This allows the car to run higher boost but isn't the greatest for fuel economy....

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Well...

  • You want the largest possible throttle opening (on naturally aspirated cars anyways) to reduce the vacuum and hence pumping losses, however, you don't want to force the computer out of closed-loop mode... So, roughly 50-75% throttle depending on the vehicle (assuming manual transmission as an automatic will probably grab a less efficient gear in those circumstances).

  • You want the RPMs as low as possible as it truly is RPMs that really burn fuel (frictional losses with higher RPMs). However, if the RPMs are too low, then you don't get the benefit of timing advance...

So, "it depends" on naturally aspirated manuals. They give you the most control of how to optimize your fuel usage, but you need to know your specific car. :-) The easiest car to optimize for is the turbo automatic... Drive as slow as possible and hope for the best since you don't really have much control beyond that... :-)

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"Drive as slow as possible and hope for the best" haha, like that's happening. XD My is SMG I wonder how that plays into all this... –  stoicfury Oct 23 '11 at 3:25

Basically, yes lower rpms == lower consumption but there is a caveat Looking at what your throttle is doing, if you are at low rpm in a high gear but your foot is on the throttle heavy then there is more air coming in and at low rpm valves are open longer so the computer responds with the right amount of fuel for the air you are sucking up with the wide open throttle. You want to get your rpms low but not at the cost of having to open up the throttle. So there is a limit. So pick the highest gearing that a light throttle can handle.

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Incorrect. Just because the throttle is wide open doesn't mean that there is actually more air coming in. Only as much air comes in as the engine can pump out. Only as much fuel is injected as is necessary for the air coming in. –  Brian Knoblauch Oct 24 '11 at 13:35

More intuitive, less scientific reply. Fuel economy of my motorcycle (250cc, electronic ignition and FI, thus comparable in behaviour to EFI automobiles), recently dropped off fairly considerably (below 80 with lowest of 76 MPG per fuel-up). Disregarding possible negative influence of winter additives (is there any truth to that?), one thing in common between those less-efficient fuel ups was me lugging it, i.e. shifting gears early, and slowly accelerating.

However, even though I took it easy, the amount of work it had to perform turned out to be higher than normal. Last two fuel-ups I went back to manufacturer-recommended shifting intervals, and my fuel economy went back over 82MPG (85 latest), and with much sprightlier acceleration.

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The answer is complicated. Lets say we have two conditions - low revs and high injector times, and twice the revs and half the injector times. In both cases the fuel burned will be the same. This is normal - we need work done and this comes from the fuel. We have to account something else - when engine turns there is friction - the more revolutions will produce more wasted energy. So we want low revs/high. But thats one side of the story - when low rev/ high torque situation is present, there are more vibration which is again wasted energy and not comfortable at all. Some of the peripherals as water and oil pump made to work at high rpm may not be efficient in low rpm , and not enough oil may be present, in this high torque situation leading to even more friction. The alternator has its own efficiency dependent on rpm. Balance is everything - do not force the engine with 1000 rpm , but do not go past half the engine speed if economy is the desired.

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