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Reduced mileage at high altitude

About a year and a half ago I moved from Chicago (500 feet above sea level) to Morelia, Mexico (6,200 feet above sea level). My gas mileage on my 2000 Ford Focus has plummeted, I used to get anywhere from 22 to 26 miles to the gallon around town in Chicago but I am lucky to get 14 or 15 here. Is it simply a function of altitude? I have a good mechanic, keep everything tuned up and in good shape. Any tips from anyone?

It could alos be the quality of the gas…and how’s the quality of the roads?
Oh, and it might help if we had the make, model, engine, and mileage of the vehicle.

It’d also be a help to know how different the climate is. In cold weather, the engine uses more gas.

If anything, I would expect an increase in mileage at altitude. I used to fly B727s for an airline. At cruise, our Pratt JT8Ds would burn 3000 pounds of fule/per engine/hour.
When landing(reduced power) 3000 pph on the fuel flow gauges would be typical.

Is your driving in Mexico on the level or are you going up and down hills?

I used to lived in the high desert of Arizona. The gas was 85 octane and my mileage always suffered. I think it’s normal even in Mexico.

Tom, one can’t compare the two.

In a car engine, the major influence of high altitude on a naturally aspirated engine is the lower air density as a component of the fuel mix. In modern engines where the air to fuel ratio is managed as a function of the amount of air entering the intake, that limits the amount of fuel as well, and reduces available power. A secondary effect of high altitude may be the temperature. Colder temperatures cause richer operation.

In a 747, aspiration is via large volumes of air entering the intake cowling at high speeds, and the turbine engine is designed for large volumes of entering air and constant high RPM operation. Its major opponent for fuel efficieny is aerodynamic drag on the aircraft. That makes altitude critical. Temperature, not so much.

All other things being equal (temp, fuel quality, flat vs. hilly terrain etc) moving to a higher altitude will increase your fuel mileage because of what TSM said - the computer pulls back the amount of fuel entering the cylinders as the amount of air entering the cylinders is reduced by the thinner air at altitude.

Of course, other things are rarely equal - for one, generally once you get that high, you’re doing at least some driving in mountains, which means going up and down hills, which uses fuel at a different rate than a flatlander will.

But, really, I think TSM got it with his first post - you’re in Mexico now. There’s no guarantee that the gas you’re getting there is of the same quality as the gas you were getting in Chicago. That plus the fact that you are right in the middle of the mountains means your gas mileage is gonna suffer.