Comment by alkonaut

I think there are still issues with even otherwise "safe" modifications. The road tax for a specific car model is (or should) be set based on emissions. Just like you would be fined if you were pulled over and had left your catalytic converter at home, it could be considered illegal to modify the fuel maps of a car to a higher power one, for example (At least if it hadn't been inspected and its tax adjusted after the modification).