Gas mileage and high altitude

My 2007 AWD Saturn Vue has been loosing gas mileage this past year, except when we drive in the mountains of Colorado. The difference can be as much as 6 mpg. What could be the cause of this difference in gas mileage between driving at altitudes of 5,000 to 7,000 feet and 7,000 to 10,000 feet? I use regular gas in my car driving in Colorado.

Engines lose efficiency at high elevation, so fuel mileage usually decreases. Short-term increases can be caused by wind or other driving conditions. The I-70 weekend traffic circus makes accurate mileage measurements almost impossible…But tailwinds and long downhill glides can increase your mileage significantly…

there was another posting about this a month or two ago; what i said was that my experience was the same as yours, better mileage at higher altitudes. main factors as far as i can tell: thinner air has less resistance aerodynamically especially at freeway speed, and less gas is being injected to maintain proper fuel/air ratio. power is clearly affected but apparently the gain in mileage more than offsets the extra push on the pedal. when i lived a mile high in boulder and would drive to ohio to visit family, the mileage would very consistently go from 35 mpg to 25 mpg, gradually as i crossed the great plains. more power, for sure, with the thick air of 800 ft of elevation but i’d rather have the mileage anyday.

This condition is also widely reported on many climbs in the Himalayas. The Sherpas are protesting as they want more money to carry the extra gas:)

Of course your mileage will suffer while driving in the mountains at altitude.

They don’t lose efficiency, they lose power. An engine produces less power at a given engine speed at high altitude, but it uses less fuel to do so, so it’s still creating the same amount of work for an amount of fuel.

They DO lose efficiency…The effective compression is lower so in order to maintain a given speed or rate of acceleration a larger throttle opening is needed resulting in lower manifold vacuum and higher fuel consumption… But you are right, much of this is off-set by decreased aerodynamic drag and the computer maintaining the proper air/fuel ratio while it nudges up the ignition advance. So perhaps it IS possible to get higher over-all fuel mileage at high elevation…But the uphill segments really destroy fuel mileage unless the wind is pushing you up that hill…

Your mileage suffers because air desnsity descreases as altitude increases. But your cylinder volume stays constant. As Caddyman pointed out, that means you’re able to draw less volume (fewer molecules of oxygen) into the cylinders, the effective compression drops, the ECU compensates by reducing the amount of fuel added (to keep the mix proper), and you thus have less fuel and less energy with each power stroke. That means that to keep the same speed you need to open the throttle a bit more.

However, that does not answer the question of why your Vue is “losing gas mileage this past year” if your driving conditions haven’t changed. When was the last time you had a tuneup including filters?

Or am I misreading the post?

Uh, no. Because there’s fewer air molecules passing through the throttle body, you have to open the throttle valve more to get a given manifold pressure than you would at lower altitude. The engine doesn’t know and doesn’t care whether that manifold pressure is due to 25% throttle at sea level or 35% throttle at 10,000 feet (or whatever the actual numbers may be). The number of air and fuel molecules that are getting burnt to achieve a certain power output remains the same, it’s just that the engine becomes limited by how many air molecules can pass through the intake system.

My wife’s Honda Element gets some of its best gas mileage ever when we go to New Mexico to go skiing. Taos, NM is about 8000 ft above mean sea level. We got 29.5 mpg there. Back in central Texas, around 700 ft above sea level, the gas mileage drops back down to 22-24 mpg.

The thinner air takes less horsepower to push out of the vehicle’s way and in addition, also effectively reduces the engines displacement, causing it to operate with the throttle more open for lower pumping losses.

There’s a good reason that airlines climb up to around 30,000 ft before leveling off and cruising.

I seldom like comparing passenger cars to race cars and like even less comparing passenger cars to turbine powered aircraft, a true apples to oranges comparison. Not scientific but I propose if the reduced density of the atmosphere at 7000ft (and it is reduced) was great enough to affect the gas mileage by reducing aerodynamic drag I propose it would also be great enough to reduce the sutability for human life.

An ICE will be more efficient at altitude (well, altitude that automobiles can obtain) because the thinner air will result in a less powerful engine overall. At 7,000 ASL, the air is only 75% as dense as at S/L.

Thus, one accrues the efficiency advantages of driving a less-powerful car (specifically, wider throttle settings resulting in less “pumping losses” for any given hp/rpm combo).

Additionally, the air, being less dense, presents less aero drag.

People can live at 7,000 ASL because a living body (unlike a machine) can engage in compensatory actions to partially counter the effects of altitude. However, a very ill person might not survive the transition…

Finally, ICE airplanes DO get better economy at higher altitude…it’s just a question of balancing this gain vs. fuel consumed to “get to altitude,” as well as potentially higher headwinds.

[P.S.all this applies only to engines capable of adjusting mixture to the thinner air. Simple carbs–and other “open loop” engine management systems–will waste fuel running rich unless re-jetted, or similar.]

You did not address the base question, I will restate, If the change of the "make up’ of the atmosphere (in both density and oxygen content) was great enough to be responsible for a 6 mpg gain in mileage, would this amount of change (assuming that it did occur) be so great that the human body could not adapt? I say YES.

I am using this comparison as my proof. A human could not adapt to an atmosphere that was changed so much by altitude that it caused a 6 mpg gain in mileage for an automobile, it would be out of the range of the bodies ability to adapt, or so I propose.

I propose to gain 6 mpg due to less aerodynamic resistance you would have to be driving in a “half atmosphere” condition. Neither the car nor the person would run, but both the car and the person do run.
In short, I don’t feel the OP gained 6 mpg by driving at altitude.

  1. Atmospheric pressure at sea level (on an “average” day) is 29.92"Hg. At lower altitudes, pressure drops off at a roughly constant 1"Hg/1000’. Thus, air pressure at 7,000’ ASL is 23/30th, or 77%, of sea level pressure.

  2. Aero drag drops off proptionally to pressure (as a car is a reasonably “bluff body,” I think we can ignore Renyold’s Number here).

  3. shows the highway MPG of an '07 VUE to be 25 mpg. Assuming 80% of drag at highway speed to be aero, one would expect a [(25/0.77)-25]*0.8 = 6.0 MPG improvement, just due to drag reduction.

  4. The highest permanently-inhabited city on earth is the Bolivian city of Potos?,at 4,090 m (13,420 ft) above sea level. The air there is rougly 60% as dense as at S/L. I have climbed a “fourteener” out in Colorado without benefit of oxygen. Don’t under-estimate the human capacity to adapt!