We have all seen the car for sale add “all highway miles” if this were always true the highways would be packed 24-7. True highway miles put less of a strain on a vehicle but I think it is the transmission that benefits the most. No shifting in and out of gear as in town and with lockup it is even less. In prior years it could be said that the even speed causes less engine wear and that is true but with 4 lane highways and greatly increased traffic there is an increase in changes of speed putting additional stress on the engine. However it is true that extended trips do put less strain on a vehicle.
Engines are designed to cruise at around 50 to 75. With the smaller engines this decreases some and remember don’t overdrive. All vehicles have an ideal cruising range some of the smaller engines this may be in the 60 MPH others may be higher.
One item that highway driving helps to decrease wear and tear is the brakes however a few panic stops from 70 MPH can decrease this in a hurry and also warp your brake rotors.