Does fuel economy get better with altitude?
 

Has anyone tried to test if you get better fuel economy at higher elevations? In theory it could have an effect since the air density decreases and you would have less drag. I know aircraft fly at higher altitudes to increase their range. It might be hard to see any difference in a vehicle since you have to control for road and traffic conditions. Plus rolling resistance is a lot of the problem at lower speeds and this doesn't apply to aircraft.

I have not tested it for lack of practical means to do so, but it should have even better benefit in a car than in an airplane.

1. Less air resistance
2. Engine detuned a bit for cruise
3. as for advantage over an airplane, a propeller will "slip" more in thin air whereas a car does not experience such slip.

Regarding aircraft mileage and altitude.

All aircraft engines (reciprocating, turbo-prop & jet) maintain approximately the same air to fuel ratio at all altitudes. As an aircraft climbs there is less O2 air molecules per gulp therefore less fuel is consumed (ratio remains the same). At +/- 18,000 ft. the amount of O2 available is 50% compared to sea level (on a standard day) thus fuel burn decrease approximately 50%.

Less O2 also means less power unless you are turbocharge. Less O2 also means less air meaning less resistance BUT it also means less horsepower, shaft horsepower or (jet thrust) available.

All other thangs being equal, humid air will result in less horsepower because the moisture molecules will displace O2 molecules. This gets into DENSITY ALTITUDE problems, see Density altitude - Wikipedia, the free encyclopedia

If you want really good gas mileage move to Leadville, CO. or Tibet.

Unless you are driving a NA Diesel in which case you will get much worse mileage at higher altitudes and more MPG at lower.

If you are driving a NA Diesel, I doubt if you could get up to Leadville, Co.

bombloader -

There's a dude with a Ford ZX2 that got monster MPG in Colorado at maybe 10K altitude. A lot of his MPG was from empty roads + NHRA background leading to great aeromods. But, I would argue that one component of his MPG came from how his ZX2 reacted to the higher altitude.

CarloSW2

I notice a definite mileage improvement when driving at 2100 ft elevation in the NC mountains when compared to driving at 500 feet elevation in the SC piedmont.

Yes. I got really good gas mileage with a WJ Grand Cherokee at 6000 feet

Aerodynamic drag is some 20% lower in Denver than in Miami. However, there tend to be a lot of annoying hills and mountains at those elevations. Some are steep enough to cause you to waste energy braking, and even small hills interfere with urban planning. I use more gas and drive more miles trying to get around Pittsburgh than Buffalo.

High altitude would be good for fuel economy except for one thing: uphill grades.

Most high altitude terrain I've ever seen is either uphill or downhill, rarely flat.

Remember you road load equation: 20 lb load per ton per percent grade. Entropy assures you never recover all your potential energy coming downhill.


BTW, a 6.2 GM diesel (mine) made it to Leadville and over every pass I tried. I even got the thing to the summit of Pike's Peak - Altitude 14,406 ft.

Caveat: I did have to slow down and it smoked like a steam locomotive.

Thanks guys. I actually understand pretty well how it affects aircraft. I know in theory it should have some effect on a car, but I also know that its really hard to test because of the aforementioned problems such as more hills in high elevation places. Just wondering if the difference was every noticeable, or if it was small enough that it got swamped in other effects. Sounds like at least some of you have noticed though. Gonna take everyone's word for it for now, because I don't feel like A-B-A testing the whole thing by making a few runs in Denver:) BTW, why the suggestions that some diesels would be different? Only difference I can see is that most of them are turbocharged, so you wouldn't lose power at altitude. Still, less drag=less power required to maintain a given speed.

I'll throw my two cents in since I live at about 5000ft: on the roads here I can average 34mpg in my GP, up and down the passes. Take a look at my gas log. I don't update it anymore, haven't the time really. But you'll see that between here and Coure De Alene I can get 34mpg with a fully loaded car, up a few thousand feet and down. When I first bought the car and drove it back from OR it managed 34mpg between the west side of Portland to Coure De Alene, so through big city traffic. I wasn't doing any hypermiling tricks, just set the cruise at 70mph and go. Come to think of it, I drove it to CA just after buying it, and averaged 32mpg over those 2000 miles, with lots of go pedal.

So my experience isn't that great of a definition. I can get the same at sea level to 6500ft. Granted those two experiences were over 2 years apart, but hey. I think the only corrolation I find is at lower elevations the car really shows its full power, which is quite fun. Its almost anemic up here, so I tend to "enjoy" it more down there, sporting with traffic and all. Plus there's the whole how much throttle needed, amount of air being sucked in, blah blah the normal stuff.

Bottom line, yes, the car gets better mileage at higher altitudes, BUT (big big but here) it's such a gas to drive down there and so boring up here (even more so when I was in the high lands of CO), the amount of power available for a given throttle input, equates to crap mileage down low and average to good mileage up high.

hmm. long post. And all I said was I love using the go pedal. hmm. At least I keep it above the 30mpg floor!

I didn't have a mpg gauge at the time, but when I towed my camper through the Porcupine mountiains in the UP, I noticed it was a lot easier to maintain 55 mph the further up I got, plus I was going uphill. A lot easier than any of the flat roads at lower elevation. Only reason I could think of was the air was thinner and less wind resistance.

Not only does power drop with altitude but engine efficiency does as well. It all comes down to whether the drop in aerodynamic drag is greater than the drop in engine efficiency. I would tend to doubt this is the case though.

38 to 44 mpg with altitude
 

Friend and workmate Steve relayed an account of Mike Sizer's Honda Civic ( late model ) which was returning 36-38 mpg HWY,then on trip to Yellowstone National Park and back,achieved up to 44-mpg in the higher elevations.Mikes no slacker ( professional musician and Air Traffic Controller @ Ft.Worth Center ) and I would not discount his claims.He accounts thinner air and ability of electronic fuel injection to compensate,for improvement.

I don't think there is a drop in efficiency. Why would people run hot air intakes if thin air didn't mean more efficiency?

Coincidentally Yellowstone is where I got good gas mileage with the Grand Cherokee. I worked there as a GIS intern in 2006. Even with all the hills, I got better gas mileage than I got in Houston.

better
 

Looks like all the gee-whiz electronic gizmos are really a good friend and ally.

Yeah they are. With an instant mpg readout, you see how much your mileage drops going uphill, and you reduce throttle accordingly.

Are you referring to the case of carbureted engines? Most non-aviation carbureted engine have fixed jetting for mixture, so yeah efficiency does decrease because the mixture becomes richer. You have to rejet the whole carburetor to fix the problem. Modern injected engines maintain a stoichometric mixture constantly, so that wouldn't be a problem.

Trapped mass due to egr plays a role as well. At part throttle at low altitude you have more egr in the cylinder than you would at a higher altitude forthe same amount of fuel burned. The egr absorbs some of the heat of combustion that would otherwise be transferred to the engine block. This allows that heat to be converted to work instead of being dumped through the radiator.