In some places cops will park at the bottom of a steep grade and watch for black smoke coming out of motor homes and pickup trucks. Black smoke usually means that they're cheating and using off-road diesel. I hope they fine the hell out of these people. Off-road diesel is meant for use in tractors, farm equipment and off-road construction equipment only. Its cheaper because there's no road tax added. If too many people cheat the system they'll end up ruining it for those of us that have a legitimate use for it.