# Elevation and Gas Mileage

Please excuse me if this has been dealt with before/recently, but as a guest I don’t have the ability to search the board. Anyway…

After a quick Google, it appears at first glance that driving at lower elevations results in better gas mileage because more oxygen = more complete combustion of fuel. However, isn’t a main factor in highway mileage air resistance? Is there some sort of “break even” point where it becomes more efficient to drive at a higher altitude because there is less air resistance? Or am I just being stupid, because the amount of oxygen is a much bigger factor than air resistance?
Confusing, I know. Sorry.

Rediculous to propose that there is more oxygen in the cylinder at lower altitude. For a given power level there is almost no difference in the amount of air (thus O2) in the cylinder. You just need to open the throttle more at higher altitude to let that much mass through.

With fuel injected motorcycles (motorcycles are quite high drag) fuel economy noticeably improves at high altitude.

In addition to reduced aerdynamic drag, the engine must be operated with the throttle further open which reduces pumping losses from the (proportionally) very large engine.

Altitude throws the mixture out of whack on carbureted engines, so unless the machine is re-jetted, economy usually degrades.

At higher altitudes gas staions have lower octane gasoline, as well.

My father, a Mech Eng for 45 years and I discussed almost this subject this past weekend (Discussing my under-powered car & living in Denver).

He remembers, sorry, this’ll be my only cite, that there’s a 33% loss of horsepower at 12,000 feet altitude as compared to sea-level. He notes that the relationship is “nearly linear”.

This relationship is due to oxygen content differences.

From here, air pressure at 10,000 feet is 71% of sea level and (from here) the air density at the same altitude is similarly 70% of sea level.

I’ll leave it to you stokes law folks to compute the implications of that.

Actually found another reference that gives that 10,000 foot power as 74% of standard. (It’s directly proportional to air density). That gives the 12,000 foot power of 69% so Dad’s numbers are pretty close.

-B

Is this because of the lower oxygen content, or the fact that the altitude messes with the carburetor? Or is that the same thing?

From here:*
The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine horsepower and torque. For tweaking the fuel/air mixture, the air density is the most important consideration.*

So it appears to be the oxygen content (as directly related to air density).

The density, by the way, is inversely proportional to the delta in temperature (cold air is more dense), but that’s Temperature in Kelvin so that proportions are comparitively small for common human conditions.

I remember traveling to Colorado years ago (70s) and having to get the carburator re-tuned for the increased altitude. The air was so thin that the car wouldn’t start above 10,000 feet or so. Having a can of whoop-ass (aka: ether) was a help. Modern oxygen sensors have pretty much eliminated this need to re-tune. I called the dealership when I moved to Denver and asked and they said I should fine just as-is - no re-tune needed.

Cooler air is also more dense. Drag racers do noticably better in the mornings when the air is cooler, and have a noticable drop off in performance at any significant altitude, such as in Denver.

Wind resistance is also lower at altitude, but I think it’s probably overwhelmed by reduced oxygen intake.

Bolding mine. The ether helped you, or the car? Sorry, Fear and Loathing is still fresh in my mind.

But seriously, thanks. Living in CO all my life got me wondering: am I getting a good deal, or getting screwed thanks to those big-ass rocks to the west?

That reference is for aircraft, taking into accoung adiabatic cooling. For land vehicles, the temperature is nearly always greater, and often MUCH greater than “standard”, thus density drops much faster climbing a mountain highway than it does climbing to the same altitude in an airplane.

Pilots refer to “density altitude” …how much take off performance you lose on a hot day. Note that the airport in Phoenix is sometimes closed to departing aircraft, while the airport in Albuquerque, at over twice the elevation, never has been. (at least in my memory) Peak temperatures in ABQ are about 20F lower than PHX. Of course that ABQ has one seriously long assed runway is a factor as well.

Once you get 1-2000’ AGL (above ground level) the temperature is much closer to the “book” value.

The OP was about fuel economy, not performance. The reduced performance at reduced air density is analogous to installing a smaller displacement engine. Considered in this way it is not at all suprising that the more economical condition would lead to a loss in performance.

Or stated another way, if you’re getting more air, you can burn more gas, so you are making more power, but then your burning more gas.

See, I assumed an opposite. Lower available power means less bang per unit gasoline. I’d argue that you’re burning the same amount of gas but getting less horsepower for your money.

Power is torque times speed after all and if speed & torque are constant for two comparable altitudes then power has to be the same, too. If the power per unit gasoline is lower due to altitude, then you need more gas units to achieve the same total power output.

Unless you’re driving a proportionally slower amount, you’d have to burn more gas to shove your vehicle to the same speed as you would lower in altitude.

The OP is also asking if the reduced air density would offset this additional fuel need.

By Stoke’s law, velocity is linear with the density of the medium you’re moving through and it looks like power is linear with the density as well. (Temperature being constant, ideal gasses, etc).

If air resistance was the only resistance to driving that I think it would be a wash. However, there’s rolling resistence of the tires, heat loss, and all sorts of other losses that are near-constant regardless of altitude.

This leads me to believe that reduced fuel economy due to lower air density would not fully offset the gains from the decreased wind resistence.

Belrix,

With a car with a fancy computer and fully functional sensors and fuel injection, the engine should be able to maintain the stochiometric ratio and continue making its 15 HP per hour per gallon [example number].
It’ll just be injecting less fuel for a given quantity of air intake.
I would imagine you’d get the same gas mileage, unless the air density provided an aerodynamic adantage.
If anything, it seems like the higher altitude would make no difference at speeds below 40 MPH [where aerodynamics isn’t a major factor] and would providean advantage at higher speeds.
If your car is underpowered, you might see some gas savings at interstate speeds, especially if you usually go 9 MPH over the limit; you car would burn less gas because you wouldn’t be making as much horepower, and thus you wouldn’t be speeding as much.

Also:
Aircraft at 10,000 feet are usually at a wide open throttle. Therefore the mixture needs to be leaned out to maintain the proper ratio. (up and down way too often to try to re-jet the engine in flight)

Auto’s very rarely run at 75% of total rated power continuously and now days the computers and FI keep the mixture ratio in the proper spot.

So add that to —

If you tried to drive with the same acceleration figures you do at lower elevations, you would be needing much larger throttle settings to achieve the same results.

In a steady state condition, it takes x HP to maintain it, the effects of wind resistance may cause a slight increase in MPG. Assuming near correct fuel - air mixtures though out

Aircraft are much more affected by this than auto’s and so the higher you go, the better off you are up to a certain point. ( wind direction must be left out for this to hold true most of the time.)

I’m going to throw a curveball in here, and talk about my car, a Chrysler PT Cruiser GT. It has a turbocharged engine. Thanks to the fancy computers and all that are attached to it, the turbocharger actually varies the amount of boost it provides at differing altitudes and densities. The result is that no matter your altitude, you have roughly the same amount of power available as at sea level.

Some “tuners” were really disappointed when they were able to run a 14 second 1/4 mile in a high altitude area, thinking their car had much more power than rated, only to discover that at low altitude areas, they still run 14 second 1/4 miles.