Comment by Arainach
16 hours ago
One person's "controlling their own engines" is another "spewing nitrous oxides, carbon monoxide, and other pollutants into the air, giving cancer to neighbors and destroying the atmosphere". We tried the "don't regulate" path and it ended in a multitude of disasters.
You can regulate emissions without preventing custom tunes
In practice, no, you can't. Certainly not without enormous costs such as mandatory regular vehicle inspections.
I don't think "enormous costs" is a strong argument here. The annual MOT is usually under £50, and that includes a fairly comprehensive roadworthiness check and emissions check. A stronger argument would be that a car owner could change some settings on their car specially to pass the emissions check, and then set them back to dirty afterwards each year.