Object: to obtain maximum gasoline efficiency for the vehicle.
I know that it’s best to keep a steady speed and use the brakes as little as possible. (Brakes turn momentum into heat and brake dust.)
Too slow a steady speed is bad… beginning from idle, going slightly faster increases the distance traveled more than it increases fuel usage. At some point air resistance becomes turns this around and further increases in speed reduce efficiency. The point at which this occurs will vary depending on many factors (such as the aerodynamic efficiency of the car) but I think it’s somewhere in the area of 55mph.
Clearly, accelerating as hard as possible to 55 would be counter-productive. But it’s not obvious to me whether accelerating very slowly is more or less efficient that accelerating at a brisk pace, with the engine in it’s maximum efficiency band. I’ve always heard that the gentle acceleration is best, but “maximum efficiency band” by it’s very name seems to cast doubt on this.
Thoughts?
There are two components that your car’s engine must overcome. First is drag and that is mostly aerodynamic but also mechanical from tire to road, bearings, etc. Second is acceleration of mass.
For starters, what gives you the idea that 55mph is the most efficient speed? There will be less drag than 65 or 75mph but there is nothing particularly magic about 55. Aerodynamic drag is a function of the cube of velocity. Going slower will always have less drag. That doesn’t mean the best speed is the slowest one. The fuel flow of your engine compared to speed and load is almost certainly not a linear function. There are also limiting factors such as available gear ratios. You’ll need to get a fuel flow gauge to determine this.
Slowest accelerations takes least energy but you have the issue of using a limited number of fixed gear ratios so the overall efficiency for miles/gallon may not be best with absolute slowest accelerations. Get an in car accelerometer (and of course a fuel flow gauge) if you want to test this yourself.
My Passat has a gauge on the dash that tells the current MPG continually. Pretty neat. My experience is that a hard acceleration drops the MPG to about 5. If I speed up gradually I can see the MPG increase with each gear shift. The optimal acceleration is quite gradual, one that would give everyone behind me a complete fit of road rage. The best milage, of course, is going downhill, but on level roads it gets 30 - 40 MPG at a steady 40 - 45 MPH. After that it drops off. When I am on an empty road, I try to keep a constant slight pressure on the gas pedal and let it speed up going downhill and slow down going uphill, as long as the speed is not unsafe.
Accelerating at 5mpg for 5 seconds takes the same amount of fuel as accelerating at 30mpg for 30 seconds. What counts is not the momentary mpg, but the average over the whole trip.
On most cars you’ll get best mpg by accelerating at full throttle (or close to it), shifting relatively low (about 1/2 of redline), then getting into the highest gear you can at cruise. Most cars get peak mpg in top gear at about 40 mph. All of this depends on the particular characteristics of your car’s engine, gears available, type of transmission, etc. If you are driving an automatic, you can generally force an early upshift during acceleration by momentarily lifting the throttle. Accelerating like (automatic or manual) this makes for a lot of movement of the throttle, and is 180 degrees apart from the “imagine there’s an egg under your right foot” advice in the '70s.
Do you mean flooring it until you get to 40 mph? What about local traffic (I think the OP assumes highway driving)? Surely, this can’t be good for local traffic?
I find my best efficieny at going around 55 mph, and not breaking until I need to, and at that, at an even keel.
(my owner’s manual theorizes that between 45 mph - 55 mph is the magic number, and that excess speed (they have a chart) above that will reduce mpg efficiency, in addition other factors like cold temps, wind, etc.)
You missed half of what I wrote. Floor it and shift at about 1/2 redline. Floor it isn’t quite right, either, as you’ll be constantly moving the acclerator up and down to shift (in a manual) or to force a shift (in an automatic), but when its in gear and below 1/2 of redline, yes, floor it.
Local traffic won’t notice any difference as your rate of acceleration will be similar to everybody else, you’ll just do it more efficiently. Local conditions matter, of course. For instance, if there is a red light at the next intersection, there’s no use in getting up to speed. If you are going up a short onramp maximum acceleration should be your goal, damn the efficiency.
Years ago, when I was a young “Jung one”, my parents would enter competitions for best gas mileage as part of the SCCA. While their road rally car was a 240Z, they would not use it for these MPG races. They would beat the small imports with a Buick Skylark. I can’t recall what all they would do to win and my searches on google for information about such events came up empty. Looking at SCCA sites may lead you to tips and tricks of the trade about these contests.
Agreed.
I know that slower is not better overall because poking along in traffic at 8MPH give horrible mileage. I know I get worse mileage at 90 than at some lower point. It seems from testing that the crossover is in the area of 55 (but maybe not exactly there, and this might vary with terrrain… hills or such. YMMV with different car, as well.)
The real question is how best to get there.
You’re drawing a conclusion without sufficient information. You seem to be assuming an acceleration rate proportional to fuel usage which may not be true. Acceleration is not velocity. If you use a given amount of fuel for maximum acceleration to crusing speed you achieve that in a short distance. If you use the same fuel to slowly accelerate to cruising speed you’ve covered a much greater distance, hence better MPG performance.
You’re reading far too deeply into the numbers. Those were just an example to point out that momentarily low milage is not necessarily a bad thing. I also know about the difference in distance, however you are missing something, too. In the 5 second/30 second example, after 5 seconds the fast accelerating car is now cruising and getting fuel economy, but the slow accelerating car is still accelerating and getting getting average fuel economy for another 25 seconds.
Accelerating to a given speed has to add a certain amount of kinetic energy to the car no matter what time period you do it in. Internal combustion engines in general will deliver energy at different fuel efficiencies for different fractions of maximum power, with the best efficiency happening somewhere near maximum power but not at maximum power. I think I read 3/4 maximum as a rule of thumb, but it would certainly depend alot on the particular engine.
No idea where the conventional wisdom of avoiding jackrabbit starts came from. It’s a nice thing to do for safety’s sake, noise, nonaggressive driving, etc - but certainly not the most fuel efficient.
The speed you drive at is a strong determinant of mileage, and I was pretty sure the most efficient speed was usually in the thirties somewhere. 55 sounds much too high.
Just putting my 2 cents in - lotta good posts already.
What I read recently in the paper as advice to reduce fuel consumption:
Accelrate at full throttle and shift to high gear as soon as possible (just as 5cents said). The reason they gave was: All modern cars regulate the fuel that’s pumped to the engine electronically. So whether you floor it, or just press half the way, doesn’t make a difference. This way you can only press too little, but never too much. It’s simply cut of at the ideal amount.
Note that fuel economy is measured by distance not time. Slow acceleration will use only marginally more fuel per distance than cruise speed so it doesn’t matter how fast you’re going at the start. Rapid acceleration gets you up to speed quickly but speed isn’t a factor in MPG.
OK, lets say you try two different ways to drive distance x. One way is to accelerate moderately quickly, using 3x the fuel per unit time that you do during cruise. You do this for 15 units of time. Then you drive at cruise for 85 units of time, and you hit distance x. Another way is to accelerate very slowly, using 1.1x the fuel per unit of time, getting to cruise speed just as you hit distance x, 150 units of time later.
Fuel consumption to reach distance x:
- (15 * 3) + (85 * 1) = 130
- 150 * 1.1 = 165
Yes, I know these are numbers I pulled out of my backside, but they do show that momentary fuel consumption isn’t necessarily important, fuel consumption for the trip is.
If you don’t believe me, will you believe Mercedes? [www.gmi.edu/admin/studaff/studorgs/technews/78-3/160.htm+mercedes+fuel+economy+training&hl=en&ie=UTF-8]This](http://216.239.41.104/search?q=cache:nYtF-UXfwQgJ:[url) article from Kettering U (formerly known as GMI, aka General Motors Institute, a well-respected university specializing in automotive engineering) discusses Mercedes Eco-training. There is one thing in the article that is unclear - the “green zone” is typically (for a modern non-turbo gas engine) between 2,000 and 3,000 rpm, and transmission gear jumps are about 33%, so if you shift up at 3,000 rpm you’ll land around 2,000 rpm, which is about where you want to be.
Articles like this one and this one (search for egg), which stem from governnent advice in the '70s, which included print advertisements that showed a foot and a gas pedal, with an egg in between (sorry, couldn’t find an image).
BTW, I’m not advocating jackrabbit starts. Full throttle with early shifts is not jackrabbit.
Let me try that GMI link again:
To anyone: how does this type of acceleration affect a turbo (added wear and tear, etc.)? What if I were to add another turbo, or perhaps a cold air blower? Will this severely negatively affect my mpg?
This doesn’t affect a turbo in terms of wear & tear, although with a turbo you’re usually better off shifting a bit higher to keep it in boost during acceleration.
Your orignial example which I was referring to compared MPG where fast acceleration at 5mpg used 6x as much fuel per distance as slow acceleration at 3mpg. Now you pull numbers “out of your backside” and make fuel used per unit of time a factor. Please correct my assumption that the definition of fuel economy was miles per gallon. How does fuel per unit time enter into it?
I’m not saying you are wrong, I’m saying you are reaching conclusions wihout sufficient information.
Should have been 5mpg vs 30mpg.
>“All modern cars regulate the fuel that’s pumped to the engine electronically. So whether you floor it, or just press half the way, doesn’t make a difference. This way you can only press too little, but never too much. It’s simply cut of at the ideal amount.”
Yeah, but “ideal” in what sense? If all the car knows is that you floor the gas pedal, all it can assume “ideal” means is that you want the greatest accelleration possible. If you’re crossing the train tracks inches in front of a speeding locomotive, it’s more reasonable to want maximum accelleration than it is to want the best fuel economy. In fact, at that moment, I for one wouldn’t mind having a bit less gasoline left in the tank…