So,thing is that if you are on roof and you go to 5th gear and rev it to redline and switch it to netural gear and turn off engine speed will slowly drop off,but after few seconds it starts to get wierd.
Let me explain.
Speed of rear wheels will be at like 180 or something and it will drop to 0 untill you update it (By pressing clutch,gas,changing gears etc) and when you update it it will return to 180. (Idk if I explained it right so here is video



(And song is Ice Cube : Hood mentality)

That has nothing to do with XRG,that's how it is with all cars. It's because the physics practically stops after a second or so when you turn off engine and don't change any controls. Probably to save CPU when it's practically not needed.