What happens as altitude increases?
Air density decreases and the output (power) of your vehicles
engine accordingly decreases as the engine cannot pack as much air
into the cyclinders to burn the fuel.
And there are other effects - air pressure decreases rather quickly with height (ACTUAL air pressure, not the number on the weather report which is extrapolated down to sea level) because there is less pull from gravity as you move away from Earth. Take dry air from the surface and push it skyward and because the air pressure drops, the temperature also drops (less pressure and the air expands, causing it to cool off). The air temperature will drop about 9.8 degrees Celsius every kilometer. And some effects, like from the 1st paragraph, you wouldn't think of. For instance, lower air pressure means water boils at a lower temperature.