Fuel Mileage - Quick vs. Steady Acceleration

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My wife and I have very different driving styles; I drive slow and steady, often taking 20-30 seconds to reach cruising speeds, and Tiffany takes off like a rocket until cruising speeds. There was a time we were sharing a vehicle (because mine was a bucket full of buttholes) and I found that the vehicle would achieve 2-3 mpg better when I didn't drive it at all. I was perplexed and taking sharp notice of anything that could've attributed to her somehow being a better driver than me. I am biased, but I found nothing. I later decided it must be because of my slow driving style. So, I have set out to recreate the scenario and perform a test to confirm the above but I cannot for the life of me be consistent enough to get any useful results. I used to have an incredibly easy drive where I never had to romp the skinny pedal but now I simply must, pretty much daily, unless I want to wait until traffic hour is over to pull out... Now that I have given this more thought I realize no one is going to have a scientific answer, if anyone has an answer at all.. but I've already typed it up so here it goes.
 
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Not the first person I've heard with this experience. If that's any consolation. When I first started learning about my cars, one of the most interesting graphs in the technical report for the 1ZZ-FE was the BSFC graph. This graph essentially shows the amount of fuel consumed per HP produced. Interestingly, it falls off moderately from idle to around 3k, then stays pretty flat to about 4k, then spikes sharply up through the redline. So that means, the engine is most efficient from 3-4k assuming a fully-loaded engine. Now lets go through the physics. It takes some fixed quantity of energy to accelerate a car to X speed. Energy is power times time. Reducing the power increases the time (as you have demonstrated) and vice-versa. The slope of that graph is linear. So now combine the power-energy graph with the BSFC graph, and you can integrate the BSFC data at a specific HP value over the corresponding time interval to give a total fuel consumption for the acceleration. I haven't run the numbers, but my guess is THAT graph will bottom out somewhere near where the BSFC graph inflects upward (around 4k). If I get a chance to run some actual numbers, I'll be sure to post the results. But this is the rough idea.
 
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Benjayman227, I must condense your scientific post down a bit: There's a point on the curve where "more gas for less time" uses less than "less gas for more time"
 
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Looking at the difference simplistically, you're spending more time in lower gears which are way less efficient than higher gears.
 
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Originally Posted by CharlieBauer
Looking at the difference simplistically, you're spending more time in lower gears which are way less efficient than higher gears.
That's a great answer and is actually true! Remember that the amount of energy required to accelerate a mass to a given velocity is independent of the rate of acceleration. Moderate acceleration to achieve cruise speed is good when the drive segment is longer. Gas engines will enrich under heavy loads and hard acceleration. Something less than full throttle is probably best. Maximizing time spent at efficient speeds is generally good, unless it's city traffic. That requires a gentle technique to achieve good MPG in a gas vehicle, a technique that is infuriating to other city drivers. In a Tesla, the rate of acceleration does not appreciatively affect range. Yes, there are some technical losses due to higher conductor resistance and slight changes in motor efficiency. But the effect is so small, a full "throttle" model S, P100D run to 60 is known to "waste" only 100 watts for the 3 seconds of acceleration, vs reaching 60mph in 20 seconds. Technically insignificant with a 100,000 watt hour battery.
 
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Originally Posted by CharlieBauer
Looking at the difference simplistically, you're spending more time in lower gears which are way less efficient than higher gears.
Aha, yes of course. It'd be similar to the mpg loss from having gigantic tires(in addition to increased rolling resistance) or installing a higher ratio differential gear set.
 
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Driving the same 40 miles one-way, for 15 years, I tested this in three cars. 2000 Outback 5-speed, 2006 audi a4 turbo automatic, 2012 Subaru Impreza w/CVT. In every car, every season, the *slowest* acceleration made significantly better MPG. By slowest, I mean a throttle opening barely above steady-state. (I drove on two-lane country roads, so could drive as slow or fast as I wanted). People confuse *efficiency* with MPG. They are NOT the same thing. ICE engines are most *efficient* at full-throttle, at mid-RPMs. That means they make the MOST HP for a given amount of fuel. But at that setting (peak torque) you are burning a LOT of fuel. Your car will burn less fuel PER MILE at the lowest throttle opening that is above idle. My cars peaked at 35-45 MPH. My Imp and CX-5 both got 45 MPG at 40-45 MPH, summer temps, no wind. It should be obvious that high-G accel burns more fuel per unit distance. You get to speed quicker, which then means you are burning more fuel due to higher air resistance, too. For entering a busy highway, gas it! Safety overrides saving money. Park in the right lane, drive the speed limit, or slower if traffic allows. In heavy traffic, draft the car in front of you (leaving enough space to avoid rear-ending in a brake-check type emergency) while driving with the pack. In the city, pull your head out of your ..., learn how to coast up to the next stop light, so you don't have to use your brakes. Don't worry about the idiots behind you, who want to drag race to the next red light. Aggressive driving in the Impreza got 25 MPG. Driving per above, 36-38 MPG.
 
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Agree 100%. My wife drives with a heavier foot than I do and let's just say this...we're averaging the SAME gas mileage in two different vehicles....but her's is a Honda CRV and mine is a full sized Chevy Silverado 4x4 V8 (both averaging almost identical MPG at 25). If she drove my truck she'd be averaging 18 at best. Nothing gets me better fuel economy than slow takeoffs, coasting as much as possible and light throttle control when up to speed. I easily get 30 mpg in my wife's vehicle by practicing light takeoffs and light throttle throughout the entire speed band.
 

AVB

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There are also pumping losses, the engine actually has to work harder to draw in air the smaller the throttle opening is. One old hyper miler trick was to use large throttle opening, but limit the rpm by short and skip shifting when accelerating.
 
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The fuel economy on modern vehicles today is largely dictated by the transmission. While driver input with the right foot does have some effect, it's the computer programming of the transmission that optimizes the fuel efficiency regardless of throttle position. The transmissions shift up quickly to keep the engine in a narrow but fuel efficient range. So it may not matter unless you are flooring the throttle.
 

AVB

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Your foot also isn't directly opening the throttle, then throw in things like vvt and variable lift, variable intake runners and eco mode.
 
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I keep mine in Eco mode. Transmission shifts faster depending on throttle input from gas pedal. You lightly push gas it is in high gear sooner and getting on interstate the difference between medium and hard acceleration changes lots of things from timing to transmission. It has taken me a while to get used to what Dodge calls intelligent transmission and it does help with fuel mileage at the cost of some driveability issues especially if you never drove one. It's all in the manual that most don't read which makes unnecessary trips to the dealership.
 
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Originally Posted by Pelican
What's the other traffic doing while you "Take your time" to reach the posted speed?
Flipping him the bird!...‚
 

AVB

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My mom's old Prius normally gets 45-50mpg with mixed driving. I had to help my dad move his dump truck last year after hurricane Micheal. The 2 speed rear end was stuck in low so 36mph was about as fast as he would drive. We drove 300 miles like that with me following in the Prius and all it would do was 35mpg at that speed.
 
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In general, I have noticed that putzing-along with slow take-offs can burn as much or more fuel as nailing the pedal during jackrabbit take-offs. This is easily seen in my vehicle by setting a mode which displays instantaneous fuel economy readings. That feature can help you learn how to economize your rates of acceleration under various circumstances. Incidentally, this car has an automatic trans that has a manual shift mode. Having driven manual transmissions most of my life, I thought I knew how to drive in the most economical way... Turns-out though, in fully automatic mode, the car's computer always gets better fuel economy than I can. I've stopped trying to beat it. Ray
 
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Originally Posted by Pelican
What's the other traffic doing while you "Take your time" to reach the posted speed?
[off-topic] ...my example: -road with 35 mph or 45mph limit. -me first at re-light -green light, i go, reach speed limit or speed limit+5 and look in my mirror: usually 2 -3 lanes of traffic are 1/4 mile behind me??? i have to say my vehicle never reached red limit, it hit 5k rpm only 3 times and usually 2.5k-3k rpm. And everybody in traffic has newer or vehicles with a bigger engine than mine... I don't get it. It was even more puzzling when i drove a base Toyota Yaris (1.5 NA engine) and ^^^ hapened.
 
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Originally Posted by pandus13
[off-topic] ...my example: -road with 35 mph or 45mph limit. -me first at re-light -green light, i go, reach speed limit or speed limit+5 and look in my mirror: usually 2 -3 lanes of traffic are 1/4 mile behind me??? i have to say my vehicle never reached red limit, it hit 5k rpm only 3 times and usually 2.5k-3k rpm. And everybody in traffic has newer or vehicles with a bigger engine than mine... I don't get it. It was even more puzzling when i drove a base Toyota Yaris (1.5 NA engine) and ^^^ hapened.
I drive like you, for the most part. I like to get out in front. I've tried to play with this line of thinking in the Volt. Being electric, I was especially curious if short, high-power take-offs to speed would result in better range than slow take-offs with a gradual increase to speed. Turns out, it doesn't really matter. Maybe a mile of range either way, but that can change by a myriad of factors. Normally I'll just stab it up to 30 or so around town, and then settle in at 35. I love being the first one at the light, the electric motor just rockets out of the hole. Not Tesla level, but still really fun.
 
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Well my vehicle is too heavy for the engine that is has so I have to really mistreat the thing to keep up with traffic. I can keep up with traffic fine but I stay in the 85% engine load(OBDII calculation) range to do it which means I'm revving thru 4500 rippems every time. I try not to care that I'm inconveniencing droves of people when I drive but the truth is that I do care. Driving on road is a major chore at this point. I can use a much lighter engine load in my wife's GX and leave traffic in the dust. It's just night and day, her car to mine.
 
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