From memory here, I think the general rule of thumb is a 3-4% power loss for each 1000 feet of altitude, figuring from sea level.
So a 300 Horsepower car at 10k feet would basically lose 30-40 HP at that altitude.
The exceptions would be on forced induction cars (turbocharged, supercharged) which can compensate for thinner air.
This is the reason many aircraft use forced induction; to offset that thin air and generate as much power as possible.