Quote Originally Posted by Sledgehammer View Post
States vary by many things. Emmissions, height of the headlights, acceptable lighting colors and distances, minimum stopping distance, tire type/tread depth, even maximum hp figures can vary. The list is quite long and each state has it's say in what it deems acceptable for a motor vehicle. For one example, Wisconsin has a maximum hp rating for cars under 5,000lbs at 800 something hp to be registered as a road vehicle. How they test for it or enforce I have no idea, but thy should give you an idea of the limits that states are able to impose.
summarising: a big mess