How much does gas mileage drop as you go to higher speeds?

[AgaBoogaBoo]

I've heard people say that the point where most people get their best gas mileage is around 55mph. Anything above, and you are going to need to make more and more power for a certain speed. (Non-linear increase)

How can I calculate what the difference would be for a car going 55mph versus 80mph in terms of fuel consumption?

 

[KLin]

1. Fill up tank, only go 55 mph on the whole tank, record # of miles, divide # of miles by gallons used
2. Fill up tank, only go 80 mph on the whole tank, record # of miles, divide # of miles by gallons used
3. compare results from step 1 with step 2.
4. ...
5. PROFIT

 

[ric1287]

that would be a pretty expensive experiment

 

[Aikouka]

Didn't the EPA do a study and find that 35 and 55 tended to be the most efficient on average?

EDIT: Probably not the EPA, but it looks like someone posted it below .

 

[miri]

I got my best gas mileage when I drove across country in Arizona where the speed limit on the interstate was 75mph. I was cruising at about 75mph in my Scion TC loaded down with all my belongings and the air conditioner on and got the highest MPG I have ever gotten in that car. It went up to about 35-36 MPG.

Earlier in the trip when I was going about 65-70MPH my gas mileage was about 32-33MPG.

And in city driving I get about 21-23 MPG in CA due to the gas and about 25-27MPG in the city in VA.

It might have been possible that the Gas in New Mexico or Arizona contributed to the really good gas mileage when I was cruising at 75mph though.

 

[spidey07]

It would totally depend on the car.

My best mileage is around 85 Mph.

 

[AgaBoogaBoo]

Originally posted by: KLin
1. Fill up tank, only go 55 mph on the whole tank, record # of miles, divide # of miles by gallons used
2. Fill up tank, only go 80 mph on the whole tank, record # of miles, divide # of miles by gallons used
3. compare results from step 1 with step 2.
4. ...
5. PROFIT

Heh, yeah, this would work. Issue is that doing an accurate test would be hard with other cars in the way that would require you to slow down or something. There are a few wide open roads where I could do this I suppose... I might sometime.

 

[KLin]

Originally posted by: AgaBoogaBoo

Originally posted by: KLin
1. Fill up tank, only go 55 mph on the whole tank, record # of miles, divide # of miles by gallons used
2. Fill up tank, only go 80 mph on the whole tank, record # of miles, divide # of miles by gallons used
3. compare results from step 1 with step 2.
4. ...
5. PROFIT

Heh, yeah, this would work. Issue is that doing an accurate test would be hard with other cars in the way that would require you to slow down or something. There are a few wide open roads where I could do this I suppose... I might sometime.

You don't necessarily need to do it for a whole tank. Just do 50 miles @ 55 mph, fill up, note the # of gallons to fill up tank, rinse repeat for 80 mph.

 

[MegaVovaN]

At the time there was a law that banned all speeds above 55mph. Didn't help, so it was lifted. Say HI!!! to 100mph desert speeds.

I think efficiency depends on the aerodynamics of the car in question. A bus would be much less effective than some small, flat sporty car.

 

[AgaBoogaBoo]

Originally posted by: miri
I got my best gas mileage when I drove across country in Arizona where the speed limit on the interstate was 75mph. I was cruising at about 75mph in my Scion TC loaded down with all my belongings and the air conditioner on and got the highest MPG I have ever gotten in that car. It went up to about 35-36 MPG.

Earlier in the trip when I was going about 65-70MPH my gas mileage was about 32-33MPG.

And in city driving I get about 21-23 MPG in CA due to the gas and about 25-27MPG in the city in VA.

It might have been possible that the Gas in New Mexico or Arizona contributed to the really good gas mileage when I was cruising at 75mph though.

I'm seeing 23-25mpg in city driving in my 91 Accord, and ~20mpg on the highway. I tend to maintain a higher speed than most others, I'd say I'm between 80 and 90 a lot of the times other than a few splurges here and there. (Speed limit is 75 so 80-90 is very common)

 

[lein]

Recently had a long road trip with a friend in a 96 Volvo (don't remember the model) that had a built in fuel economy sensor. These numbers are not perfect but I guess it can give you a general idea. cruising at 72 mph, the sensor says about 28-29 mpg. At about 78, it drops to around 25-26 mpg. A/C decreases mpg by 1-2, and drafting off an 18-wheeler (probably not a good idea) increases it by 1-2 mpg.

 

[AgaBoogaBoo]

Originally posted by: KLin

Originally posted by: AgaBoogaBoo

Originally posted by: KLin
1. Fill up tank, only go 55 mph on the whole tank, record # of miles, divide # of miles by gallons used
2. Fill up tank, only go 80 mph on the whole tank, record # of miles, divide # of miles by gallons used
3. compare results from step 1 with step 2.
4. ...
5. PROFIT

Heh, yeah, this would work. Issue is that doing an accurate test would be hard with other cars in the way that would require you to slow down or something. There are a few wide open roads where I could do this I suppose... I might sometime.

You don't necessarily need to do it for a whole tank. Just do 50 miles @ 55 mph, fill up, note the # of gallons to fill up tank, rinse repeat for 80 mph.

Yeah, that makes sense, just enough to get an accurate test.

 

[George P Burdell]

Graph

Details

 

[AgaBoogaBoo]

Originally posted by: lein
Recently had a long road trip with a friend in a 96 Volvo (don't remember the model) that had a built in fuel economy sensor. These numbers are not perfect but I guess it can give you a general idea. cruising at 72 mph, the sensor says about 28-29 mpg. At about 78, it drops to around 25-26 mpg. A/C decreases mpg by 1-2, and drafting off an 18-wheeler (probably not a good idea) increases it by 1-2 mpg.

Interesting. I've seen M3's that had a little meter showing gas mileage... wonder how hard it would be to rig up something like that and how accurate it would be. (nevermind, probably not worth it)

 

[brandonb]

I used to travel between Denver and Minneapolis (about 800 miles) quite frequently last summer. I did tests with my vehicle testing the same thing. One trip I set cruise control to 65, the next 70, and the next 75, and the next 80. I tested to see the difference in MPG and trip duration. My car has a computer so its easier to calculate, as I can reset before every trip. I have a Grand Prix 3.8L supercharged engine.

I don't remember the exact numbers of every trip anymore, so these are just the general figures (probably pretty close to the actual numbers)

65 mph = 32 mpg
70 mph = 29 mpg.
75 mph = 25 mpg.
80 mph = 22 mpg.

Obviously slower is better MPG... However, I noticed that going about 70-72 mph gave you the best bang for the buck.

 

[AgaBoogaBoo]

Originally posted by: brandonb
I used to travel between Denver and Minneapolis (about 800 miles) quite frequently last summer. I did tests with my vehicle testing the same thing. One trip I set cruise control to 65, the next 70, and the next 75, and the next 80. I tested to see the difference in MPG and trip duration. My car has a computer so its easier to calculate, as I can reset before every trip. I have a Grand Prix 3.8L supercharged engine.

I don't remember the exact numbers of every trip anymore, so these are just the general figures (probably pretty close to the actual numbers)

65 mph = 32 mpg
70 mph = 29 mpg.
75 mph = 25 mpg.
80 mph = 22 mpg.

Obviously slower is better MPG... However, I noticed that going about 70-72 mph gave you the best bang for the buck.

Thanks for your numbers, I think somewhere around 65 might be my car's optimal range. I'm going to keep track of this on a daily basis now on some of my trips and see what I can come up with.

 

[natto fire]

Originally posted by: AgaBoogaBoo

Originally posted by: lein
Recently had a long road trip with a friend in a 96 Volvo (don't remember the model) that had a built in fuel economy sensor. These numbers are not perfect but I guess it can give you a general idea. cruising at 72 mph, the sensor says about 28-29 mpg. At about 78, it drops to around 25-26 mpg. A/C decreases mpg by 1-2, and drafting off an 18-wheeler (probably not a good idea) increases it by 1-2 mpg.

Interesting. I've seen M3's that had a little meter showing gas mileage... wonder how hard it would be to rig up something like that and how accurate it would be.

It's quite easy. All those things are just glorified vacuum gauges, and are quite simple to install. They are usually mechanical, but the more expensive ones are electrical. Obviously, your car's fuel system needs to be in working order for these to be even slightly accurate, since fuel economy is only related to engine load and nothing else.

 

[Syrch]

so when a manufacturer says a vehicle has this MPG rating what range are they using? Its best?

 

[Sheep]

SFGate.com did a writeup on their findings when they drove around the Bay at 55mph. Given how people drive around here, I'm not surprised they got flipped off numerous times.

SFGate.com article

 

[AlienCraft]

My Focus gets 29/31 mpg combined when I keep the hwy speed at or below 70 mph max.
When I exceed 70 and do the "keep up with traffic " mode of 75/80, it drops to 24 mpg . :Q
So,I but a red arrow sticker on the speedo dial to remind me.

 

[ballmode]

I think it depends on car, weight, aerodynamics, gear ratios, and temperature.

 

[Cogman]

to REALLY calculate how much of a difference it makes, you need to through in a bunch of equations like air friction, tire friction, engine efficiency, ect. All of which take a lot of physics (Good luck ) Your friends are right in telling you that your gas mileage does not follow a linear path. I can remember hearing that 55 is about average for best gas milage.

 

[RichUK]

I never have to worry about fuel costs.

 

[Zenmervolt]

Highest mileage is generally achieved when in the highest gear at an RPM just barely above where the engine would otherwise lug. For most cars, this is between 35 and 45 mph if you truly drive at a constant speed. However, since roads with 35-45 mph speed limits require stopping and frequent variations in speed due to curves in the road and such. Similarly, highways marked at only 60 mph tend to include many more changes in elevation and corners than highways marked at 70-75, which means that some people may see better real-world mileage at 70-75 simply because they are on flatter terrain and holding a more constant speed rather than accelerating/decelerating all the time.

Drag increases with the square of speed (double the speed and your drag quadruples) and power increases with the cube of speed (double your speed and you need eight times the power). Assuming the same gear, a car needs eight times the HP to go 70 mph as it does to go 35mph.

ZV

 

[Zenmervolt]

Originally posted by: Captain Howdy
It's quite easy. All those things are just glorified vacuum gauges, and are quite simple to install. They are usually mechanical, but the more expensive ones are electrical. Obviously, your car's fuel system needs to be in working order for these to be even slightly accurate, since fuel economy is only related to engine load and nothing else.

Wrong.

These guages have two inputs: Injector duty cycle (read from the engine control computer) and vehicle speed (either an input from the engine control computer or from a hall-effect sensor on the speedometer in the case of a mechanical speedometer). They do not measure manifold pressure.

By interpreting the injector duty cycle reading and thereby determining the fuel flow rate the guage can compute the fuel flow in gallons per hour. This result is used along with the speed input from the speedometer to calculate the number of miles per gallon. At least this is how the guages on cars that use the BOSCH Motronic 3.x fuel injection system (Porsche 944 series specifically, though several other vehicles use variations of this same BOSCH Motronic unit).

I'm sure that some of the very early guages were vacuum-driven (heck, the entire BOSCH D-Jetronic electronic fuel injection system was driven by manifold pressure), but the modern systems read injector duty cycle and speed to determine the economy.

ZV