The sparse atmosphere robs the engine of power, but that doesn't mean that it is less efficient, I think that matters on the car/driver.
Personally, high altitude driving helps my fuel economy -- I would speculate that it's due to decreased air resistance and the lack of power forcing me to drive a little more conservatively.
Personally, high altitude driving helps my fuel economy -- I would speculate that it's due to decreased air resistance and the lack of power forcing me to drive a little more conservatively.