> But a car is sold. So if you want to cut of a piece of the car, what's going to stop you?
Government regulations. In Germany (and probably also in the rest of the EU, since that stuff has been harmonized over the last decade), the tank volume is recorded in field #12 of the car's registration/title document... which means any form of DIY modification without changing the title will yield you a criminal charge for operating a car that is not roadworthy.
To my knowledge, they do not as part of the regular checkup, but for car models where such modifications are easily possible they do have the right to do so.
Edit additionally, ordinary police also has these rights, and there are, at least in Germany, focused checkpoint controls that specialize in checking for illegal / untitled modifications ("Schwerpunktkontrolle"). And in case of a severe accident where an auditor combs over your vehicle to determine fault, something like this will pop up and be sanctioned, even if it had no relationship to the accident at all.
In any case I would not go and risk jail time, a massive fine, or in worst case forfeit of the illegally modified vehicle.
In software, the trick is it is licensed not sold, so you are not allowed even if you can do something.
But a car is sold. So if you want to cut of a piece of the car, what's going to stop you?