With the electronic engine controls, you probably get more spark advance at elevation, which helps improve engine efficiency at altitude. So the combination of all that the engine controller is doing keeps all the variables at the best point for the driving conditions.
The thinner air at altitude would make a difference in principle, but I don't think you would notice any difference at the altitude and speeds at which you were driving. Air density and the associated drag makes a difference with airplanes, which is why pilots like to climb as quickly as possible.
The pumping loss/improvement resulting from lower manifold vacuum is another point that is correct in principle, but I don't think you would detect any improvement from that alone.
Harvester
