aside from the lower stresses and continuous cooling and lubrication that the steady-state conditions of long-distance driving provides, the engine wear per mile is lower for highway-driven miles because these are performed in the car's top gear, where there are less engine revolutions per vehicle mile.

there is also less opportunity for condensation of the acidic byproducts of combustion in an engine/exhaust system that is maintained for long periods of time at elevated (operating) temperatures, resulting in less internal corrosion.