Also, an engine in a car cannot run all the time at the same load, because there are hills, crossroads, faster and slower roads, etc., while a plane operates in

Also, an engine in a car cannot run all the time at the same load, because there are hills, crossroads, faster and slower roads, etc., while a plane operates in much more constant conditions, so the engine can work at roughly the same load most of the time. Combustion engines doesn't scale, so the engine cannot have the same efficiency for every possible load. (And it's not practical to put an electric generator and engines, which are much, much better, in a plane or car.)