In US, car modification is just about whatever you want to do in most regions. In at least one European country, like Germany, vehicles must use OEM type replacement parts, which is nothing more than industry protecting their turf.
Emission checks do exist over here in select areas, but vehicle inspections are not done in every state. . Vehicle inspections are more about safety aspects, not the parts installed. I've never heard of an accident due to say new shocks that weren't OEM quality, or new brakes that weren't OEM quality, but in Germany, no excuse, from what I understand via forums.






