[RedDevil]

It depends on your drive.

For a gasser the altitude should not have that much of an effect.
The density of the air/fuel mixture is primarily governed by the throttle plate. The only real difference is at idle (needs to be set wider at altitude) and WOT.
WOT at altitude would be the same as just below WOT at sea level.
At altitude you'd just push the pedal down slightly deeper.

Lower atmospheric pressure reduces pumping losses. You could get better mileage in the mountains than is possible at sea level, as long as you stay level. That's usually a problem at altitude. Steep uphills kill FE and steep downhills just waste potential energy.

Carburetors may act differently at altitude; the fuel mixture may become too lean. With modern engines and injection the ECU will command the right mixture.

For a diesel, raising the ratio could help because diesels don't regulate power with a throttle plate; and they use a higher compression ratio.