As long as the truck is not used in commerce, as in anything that generates income, you should not need DOT numbers. That's according to federal rules. Each state has their own rules that can be more strict than the federal government's so you'll have to check with them. Oklahoma is pretty lax on lighter trucks but I have seen DOT pull pickups with trailers loaded with firewood into a weigh station. Again what makes it commercial is whether it generates income, not size. Haul firewood for sale, your commercial. Help a friend move and get paid in beer, technically still commercial!
