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what speed gives you the best gas Mileage

J

jrgilchrist

NAXJA Forum User
Location
louisville ky
SO i have plenty of time and i like saveing money and i guess being safe as well, so i was wondering what speed should i go to acheive the best mileage and get the most out of every tank? i was thinking like 57.5mph but i realy have no idea
 
I was told by someone that the speedlimit is set on freeways and highways to not only be a safe limit but also is set for the most efficient speed for fuel consumption. I doubt its true...but who knows. Personally I seem to get better mileage at about 60-65 on the freeway...once I get up to 70 or higher my mileage just drops...

Most of it depends on how you drive though. If you slam the pedal down coming out of a stop your gonna get bad mileage. If you try to keep the needle under 2k rpms coming out of a stop and while on the road your probably gonna get better. Once you get to a certain speed though...keeping it under 2k is impossible.
 
If you're talking about the freeway, the minimum posted speed of 45 MPH will return the best mileage.

ymmv
 
I think the mileage was lowered back in the day to 55 to save gas but i tend to make things up alot,

Been keeping it under 2k a much as possible,

so does slower = better mileage??? beyond the obvious yes but is there ever a point where the mileage isn't better when going slow?
 
The only thing in the article I tend to disagree with is the cruise control, though this may also depend on the vehicle. I think for one thing that it's possible to drive efficiently by varying your speed a good deal depending on circumstances. In hilly country, for example, if there's nobody behind you, there's really no need to sustain your speed up every rise, and no need to hold it on the way down. I don't have cruise, but my wife experimented with her Honda and found that her mileage went down with cruise, because it was always struggling to maintain speed, and since it couldn't see ahead, it would be surprised by changes in slope and have to do so abruptly. At least on that little low-powered car, in hilly country, cruise was not the best choise.
 
Wind resistance is the biggest consumer of your energy expenditure, aka mileage.

As you double the speed, you increase the wind resistance by a factor of 4. This is a rule of thumb and may change depending on standing waves and vehicle shape, but it is pretty close. That is why the change from 75mph to 55mph made such a difference in mileage when the speed limits were reduced.
 
Last edited:
old_man said:
Wresistance is the biggest consumer of your energy expenditure, aka mileage.

As you double the speed, you increase the wind resistance by a factor of 4. This is a rule of thumb and may change depending on standing waves and vehicle shape, but it is pretty close. That is why the change from 75mph to 55mph made such a difference in mileage when the speed limits were reduced.
Click to expand...

Yes, but...

In 1974 (?),
- nary a car on the road had overdrive
- aerodynamics were not commonly used... still not on XJ's... :cool:
- engines ran with "dumb" carburetors... not modern closed-loop fuel-injection
etc.

In short... I believe that in 1974, going slower MAY have saved fuel. Though, I still kinda doubt it. CERTAINLY every car has a "sweet spot"... based on aerodynamics, gearing, etc... and it's different with every car.

With that said... even within the realm of XJ's, it will depend on your axle gears, auto or manual tranny, tires, lift, etc.

My '97 Dodge Stratus did best between 65 and 70... because it had an overworked/under-appreciated 2.0L Neon engine and 5-speed. At 55, it was simply running too slow to be efficient, had to be downshifted for the slightest hill, etc.

Den

So there is no simple answer. Not even "55". :)
 
DenLip said:
Yes, but...

In 1974 (?),
- nary a car on the road had overdrive
- aerodynamics were not commonly used... still not on XJ's... :cool:
- engines ran with "dumb" carburetors... not modern closed-loop fuel-injection
etc.

In short... I believe that in 1974, going slower MAY have saved fuel. Though, I still kinda doubt it. CERTAINLY every car has a "sweet spot"... based on aerodynamics, gearing, etc... and it's different with every car.

With that said... even within the realm of XJ's, it will depend on your axle gears, auto or manual tranny, tires, lift, etc.

My '97 Dodge Stratus did best between 65 and 70... because it had an overworked/under-appreciated 2.0L Neon engine and 5-speed. At 55, it was simply running too slow to be efficient, had to be downshifted for the slightest hill, etc.

Den

So there is no simple answer. Not even "55". :)
Click to expand...
Ive got a simpler answer...

0...engine off, in park, in your garage or driveway. Best mileage you can get from a tank of gas.
 
Matthew Currie said:
The only thing in the article I tend to disagree with is the cruise control, though this may also depend on the vehicle. I think for one thing that it's possible to drive efficiently by varying your speed a good deal depending on circumstances. In hilly country, for example, if there's nobody behind you, there's really no need to sustain your speed up every rise, and no need to hold it on the way down. I don't have cruise, but my wife experimented with her Honda and found that her mileage went down with cruise, because it was always struggling to maintain speed, and since it couldn't see ahead, it would be surprised by changes in slope and have to do so abruptly. At least on that little low-powered car, in hilly country, cruise was not the best choise.
Click to expand...
Good point, Matthew.

A very long time ago I read that the most efficient mode of operation is not constant speed but constant throttle. Pick a speed on flat ground and never vary the position of the throttle peddle. Doesn't work in New England, but it might work in the mid-west on days when there isn't a lot of traffic.

Failing that, probably somewhere around 45 to 50 MPH would be optimum. Beyond that, you lose maybe 1 to 3 MPG for each 5 MPH increase in speed.
 
To maximize mileage, we used to use a vacuum gauge. You would vary the throttle position and the gear to keep as high a vacuum as possible.
 
there is no simple answer, for every vehicle is different, you want to look at your rpm, i usually keep mine under 2000rpm and it seems to be pretty good for me.
 
Its whatever speed you are going in your highest (numerical) gear (i.e. 4/5) at the lowest throttle position while maintaining velocity. As mentioned wind resistance increases with the square of velocity (while doubling your speed will increase your wind resistance by 4, 5x the speed = 25x the wind resistance... OUCH!), however, at the same time, you want to go as far as you can per each engine revolution.

Whoever said 0 (or parked) got it wrong also (only due to a deficiency in mathmatics though ;). Although you are saving gas, you aren't getting anywhere either. It's the old, numerator vs. denominator debate when both approach 0... is it infinity or is it 0....

... ok... Nerd... out....
 
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