Many websites that give tips on increasing mileage mention (typically in passing) that you should not accelerate hard.

Examples include (from the first two google hits on "increase gas mileage")

(to be fair, The third hit does not mention this and merely comments on the fact that "Letting up on the gas often eliminates the need for braking", which is not what I'm skeptical about.)

I've also had multiple friends comment (sometimes claiming personal experience) that I should not floor to save on gas.

However, it runs contrary to my personal experience with my current car, which I happen to share with a person that has a very different driving style. I typically get better mileage while mostly flooring when accelerating, but anticipating braking quite a lot more.

From a physics perspective, it does not seem obvious why this would be the case. After all, the chemical energy from the gas is converted to kinetic energy and heat (and wasted gas?).

So... Why would the efficiency ratio significantly change if I floor the gas pedal? Does it at all? Is it engine/car dependent ?

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    Mileage varies a lot with transmission gear selection, and in an automatic transmission vehicle the accelerator position changes that. So it may depend on the specifics of your car. – Craig Stuntz Jun 17 '11 at 10:57
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    If engines were consistently efficient and put out the same power at every RPM, we wouldn't have transmissions. We have transmission not only because engine output at low RPM has low torque, but also because the engine is most efficient around 60-70% of its peak RPM. In other words, depending on your transmission gearing and ECU, flooring it may put your engine into a more efficient RPM than slow acceleration. google.com/search?q=what+is+the+most+efficient+rpm – Adam Davis Jun 17 '11 at 12:02
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    You can share your car with somebody, who doesn't have a different, individual driving style, but maybe is driving shorter distances at lower speed, driving more often in stop-and-go traffic, load more things into the car, use the heating or cooling more intensive, put aerodynamically problematic things on the roof and so on, and so on. – user unknown Aug 14 '11 at 5:01
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    Anecdotally, my mom's Toyota Camry Hybrid has the fuel economy gauge. The one time that I was late and driving much more aggressively I attained the 40 mpg mark much quicker and kept it there much easier than my normally conservative driving style. – Wayne Werner May 19 '12 at 14:37
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    Some of these factors are complicated by the fact that the emotional states that lead to aggressive driving also lead to faster driving. This, then, has an effect both because gasoline engines tend to have a sweet spot in overall fuel efficiency, and because going faster means that you're dealing with more energy loss to wind resistance per mile. – Ben Barden Oct 10 at 20:11

Flooring the pedal is fine if you make use of the energy you've consumed, and keep going.

It's a recommended technique: Burn and Coast (or Pulse and Glide) tries to make best use of the accumulated kinetic energy. Apologies for the Wikipedia link: here's another recommendation of coasting, which also points out the value of working the engine in its most efficient range.

There is a possibility that by opening the throttle too much, the engine burns too rich a fuel/air mixture and wastes fuel, but modern engine management systems usually take care of that by interpreting your throttle input as merely a request to go faster - the ECU then decides how much more fuel it should use, possibly optimizing for efficiency over outright performance.

What really ruins mileage is braking, because you're throwing away that kinetic energy you've built up and discarding it as heat.

I think the reason for the advice not to accelerate hard is that in traffic you would soon have to slow down again, in which case the rapid acceleration costs you more fuel, but brings no benefit.

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    I'd also add that aggressive acceleration tends to mean higher engine speeds - we change up gears later than when accelerating gently. If you're driving an automatic, it will shift up later when you floor it. The higher the engine speed, the more energy that is lost to friction. Given that engines are already only 15-20% efficient, I'd estimate that the difference in efficiency between running an engine at 3000rpm compared with 2000rpm is quite large. – Tim Rogers Jun 17 '11 at 11:39
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    Alas, the references you cite do NOT support "flooring" the accelerator, which is what the original question asked. I went to them, genuinely surprised that such a driving style would be efficient. The Wikipedia page talks about "accelerating to a given speed", but not at what rate. The other link suggests that "pulse and glide" is done by "accelerating at about 2/3 of maximum acceleration". – Oddthinking Jun 17 '11 at 11:57
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    @Tim It depends on the engine, however there is a peak efficiency for most engines that is higher than 1,000 rpm, and lower than the max engine speed: en.wikipedia.org/wiki/Engine_efficiency So for some engines you will actually have higher efficiency at 3k rpm than 2krpm. The friction difference is not nearly as large as you indicate - keep in mind the engine is actively lubricated. – Adam Davis Jun 17 '11 at 11:58
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    In a manual transmission equipped car, 75%-90% throttle while short-shifting (keeping the RPMs down), reduces pumping losses and can be more efficient than accelerating slowly. Going WOT is rarely recommended as the typical automotive ECU goes into "fuel dump" mode at full throttle (richer than normal mixture to minimize any possibility of engine damage). – Brian Knoblauch Jun 17 '11 at 13:22
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    I note that in the Wikipedia article section "Burn and Coast" is completely unreferenced. That means it should be treated with much more skepticism even than most Wikipedia articles. – DJClayworth Jun 17 '11 at 13:37

Yes.

...with the caveat that there are some slightly conflicting opinions.

  • TEST of various fuel economy tips by Edmunds.com:

If you slowed your 0-to-60-mph acceleration time down from your current 10 seconds to a more normal city pace of 15 seconds, you'll feel the savings immediately.

Up to 37 percent savings, average savings of 31 percent

The method, summarized below, is found HERE:

  • Cycle 15 times from 0-75mph at 3/4 throttle, braking hard in between, total: 55 miles
  • Cycle 15 times from 0-75mph at 1/4 throttle, braking lightly in between, total: 55 miles
  • Cycle 25 times from 0-65mph at 3/4 throttle, braking hard in between, total: 25 miles
  • Cycle 25 times from 0-60mph at 1/4 throttle, braking lightly in between, total: 25 miles

Now, I really, really, really wish they had isolated their variables. I don't know why they considered it necessary to vary the braking style and top speed in these cycles. There's at least some indication here, though, that accelerating faster decreases fuel economy. I consider "flooring it" to be a subset of "accelerating faster."

  • THIS 2001 paper from Energy and Environmental Analysis, Inc., on which the US Dept. of Energy bases it's fuel economy recommendations HERE tested 17 cars and the effects of "aggressive driving cycles" (faster acceleration/deceleration, different maximum speeds, etc.) and concluded that:

Very powerful cars exhibit negligible fuel economy penalties, while an average car is likely to experience a penalty of about six percent [at lower speeds]. At higher speeds, typical of urban expressway driving, however, the fuel economy penalty of aggressive driving is both significant in magnitude and more consistent across all cars. The average car is likely to experience a penalty of 33 percent, with more powerful cars experiencing a somewhat lower penalty of about 28 percent.

So... there was a loss due to aggressive driving, but it's not that high and doesn't square with what Edmunds.com said, either (Edmunds.com had much higher loss reports). This paper also didn't isolate all variables, but went with a "driving style."

  • THIS paper by Dr. van der Voort, looking to design a dashboard device to provide driver feedback in order to increase fuel economy. HERE is a layman's writeup about his summary from The New York Times.

From The Times' article:

"People were shifting too late from first to second, and from second to third," Dr. van der Voort said. People saved the most gasoline when they pushed down on the accelerator briskly and then shifted quickly, keeping the revolutions per minute low -- not by accelerating very gently.

And from van der Voort's actual paper:

Further analysis revealed that drivers without support [the dashboard display instructing them on optimum driving habits] shifted significantly more times too late from 1st to 2nd gear and from 2nd to 3rd gear than drivers who received support (and drove more fuel-efficiently). No significant differences between the groups were found with regard to shifting from 3rd to 4th gear.

So, we can see that late shifting results in a decrease in fuel economy. In automatic transmission cars, "flooring it" will result in the car being kept in as low of a gear as possible, thus bypassing the optimum shift point for maximum fuel economy.

I wish the sources had been as simple as "faster acceleration yields lower fuel economy," but it wasn't quite that simple. For instance, I was quite surprised to find that more powerful cars did not suffer as badly from aggressive acceleration in the second source! The overall convergence of the sources is that faster acceleration (or at the very least, not shifting as speed increases) yields lower fuel economies.


I really wanted to answer this in terms of power curves and torque, but could not get my head around enough to put the facts together. I think the real why of this answer would lie in keeping the power output of the transmission (which is the torque of the engine "filtered" through your gear box) matched for the speed of the vehicle. My attempts at digging there tended to come up primarily with "hot rod" types of sites that are primarily focused on maximizing acceleration, not fuel economy.

At 100% throttle, the ECU leaves "closed loop" and enters "open loop" fuel enrichment mode. This is bad for fuel consumption. The most efficient throttle opening at which to accelerate is just before fuel enrichment mode. Typically 85% throttle is quoted as a safe amount to gain best results over at ecomodder.com where this is discussed as much length. See 'WOT' (Wide Open Throttle) discussions here:-

https://ecomodder.com/forum/showthread.php/glossary-take-ii-3505.html https://ecomodder.com/forum/showthread.php/difference-fuel-between-3-4-wot-4115.html https://ecomodder.com/forum/showthread.php/how-accelerate-28617.html

Here's another: https://ecomodder.com/forum/showthread.php/speed-vs-mpg-6th-gear-wot-24831.html "at WOT (Wide Open Throttle) the ECU enters what is called a power enrichment mode. Typically this involves temporarily entering open loop mode taking the O2 sensors out of the fuel calculation equations. Power enrichment mode is not desirable when you have fuel economy in mind not only because you are using large amounts of fuel to travel the same distance but since the ECU is ignoring the O2 sensors the engine typically ventures into a rich burning state which can be bad for plugs, cat converters, etc."

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

  • This could use more citations. In particular, a specific citation of ecomodder.com (possibly with quoted material) rather than a blanket citation of the site itself. – Brythan Jul 9 at 14:12
  • @Brythan yeah, if you can link to and quote a specific article on ecomodder.com that indicates this then this is a solid answer. However, just a blanket citation to the site and no quoting from them makes this a post in danger of being deleted. I'm not going to flag for removal for now but you're gonna want to cite your sources better as others may flag to delete. – DenisS Jul 13 at 14:45
  • And just some forums posts may not be enough 'proof'. Make sure you refer to solid date, not just opinions from random people. – Jan Doggen Oct 9 at 8:15
  • If these forums have any evidence, please actually quote it. I sampled them and didn't see any. If they don't have evidence, then this answer is just a repeat of the claim, not an actual answer. – Oddthinking Oct 9 at 13:45
  • The 1st link I provided states: Wide Open Throttle. Generally avoided in fuel efficient driving, since it causes a computer controlled engine to enter "open loop" mode, where it burns significantly more fuel than normal. The other links provide discussions in which users discuss the findings from their instrumentation monitoring fuel consumption. I'm surprised this is proving so contentious to some. – TopCat Oct 10 at 10:11

It´s a tricky question, one that can not accurately be answered without having access to the specific efficiency data of your car.

Apart from the actual acceleration energy needed for accelerating a certain mass to a specific speed you loose Energy in three other places.

  1. Rolling resistance of your drive train and treys. Let´s for the sake of the argument assume this is pretty much constant.

  2. Air resistance. This is dependent upon how wind slippery your car is designed and upon the speed of the air, see drag coefficient. Note that airspeed goes into the equation to the square. In this department you´ll lose with faster acceleration, because you will spend a longer part of your journey in higher speeds, thus needing more energy to overcome air resistance.

  3. Engine efficiency. How efficient your given engine actually converts fuel to rotations depends on its individual characteristics, the rpm and the pressure/load. A typical gasoline engine has its peak efficiency at ~ 80% throttle and between 1500 and 2500 rpm. But this varies upon a lot of factors. Now to match this to your actual road speed you´ll need to compromise, save a cvt transmission, because you cant keep the revolutions constant. (Side note: Toyotas hybrid systems optimize on this via cvt transmission, hence the wired noises with unchanging rpm while accelerating)

Here is an example of such a map - with source article enter image description here

So the Answer is: It depends, are you able to keep your engine at such a sweet spot when accelerating that you can make up for the losses in air-resistance?

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