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Thinner air at altitude means less air. That means: less fuel used , also, because the O2 sensor is trying to maintain that 14.7 : 1 air /fuel ratio. This prolly helps mileage. However when it's burning less fuel, it's also making less power. That's not necessarily a bad thing. There's just usually a trade-off between power and economy.
Mike in AR
Interesting.. then can you explain why gas mileage at high elevations DECREASES?? When I did the Dusy-Irshim trail (peaks at 9700 feet) above Fresno, it's 31 miles long and I burned up nearly 18 gallons of fuel. The carb'd V-8 (and I warned them before we got to the trail) got less than 2 miles per gallon!!
Gnarls.