[DavS1]

Barrometric pressure. at normal sea level it will be ~1bar, so if your car for instance runs 1bar of boost, absolute pressure the engine see's will be 2bar, 1bar atmo + 1bar boost.

at high altitude (or higher) the atmo pressure drops, so your car would have been seeing less boost = lower power = less fuel used as less air consumed.

I'd say an N/A car would suffer more as they only have atmospheric pressure, no boost.