Does fuel economy get better with altitude?

[ConnClark]

Originally Posted by bombloader

Are you referring to the case of carbureted engines? Most non-aviation carbureted engine have fixed jetting for mixture, so yeah efficiency does decrease because the mixture becomes richer. You have to rejet the whole carburetor to fix the problem. Modern injected engines maintain a stoichometric mixture constantly, so that wouldn't be a problem.

Trapped mass due to egr plays a role as well. At part throttle at low altitude you have more egr in the cylinder than you would at a higher altitude forthe same amount of fuel burned. The egr absorbs some of the heat of combustion that would otherwise be transferred to the engine block. This allows that heat to be converted to work instead of being dumped through the radiator.