Does flooring the gas pedal reduce mileage?

Question

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")

• Aggressive driving (speeding, rapid acceleration and braking)
• By minimizing the number of times you brake hard or accelerate rapidly, you can save on gas

(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 ?

Answer 1

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 lose Energy in three other places.

1. Rolling resistance of your drive train and tires. 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: Toyota's 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

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?

Answer 2

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.

Answer 3

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.

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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.

Answer 4

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."