In Finland you can put crazy stuff on your car but the amount of paper work it requires to be legal on the road is crazy. Basically you need to prove that the mods you did didn’t make the car unsafe for anyone.

Basically for example a coilovers kit which is pre approved is easy, custom one hard because you need to provide all strength calculation for inspection.

Factory tunes need to compile with emissions regulations so with custom tune which doesn’t compile with them you might save some amount of fuel. Basically it’s illegal but there isn’t really a way to test for example NOx easily, that’s why it took so long to catch vw with dieselgate.