Some of the on-road engine are turbo engine's they use a different type of glow plug and use a different type of head button. The turbo's tend to have more Hp on the top end but lack a little to the off-road engine's in torque. Some guys use on-road engine's for off-road use and some use off-road engine's on-road. There is no law saying you cant do this so a lot of guys do. But yes usually on-road engine's like to rev a little higher then the off-road engine's do.