I don't have any numbers to back it up, but my impression was that it varies from vehicle to vehicle. My Insight, for instance, is already near WOT in most conditions due to its luxuriously tall gearing, so taking it up 10,000ft results in needing to rev it up a lot more, hurting economy. Ditto for cars with CVTs (that have sufficiently wide ratios) or hybrids which can rev the motor independently of wheel speed. Higher powered manuals and automatics should benefit... except that most high altitude driving will probably also be in mountains.
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