Given the air pressure on Norvos, Jahtra, etc. are 1 atm, we would probably start seeing transonic effects at speeds lower than supersonic. certain areas around the car would be moving supersonically before the car itself is so the body of the car would start getting buffetted by shockwaves at around 1000kph (guesszing). This all depends on the aerodynamics of the car, and seeing that the cars are nearly blunt bodies, that transonic shuddering would be brutal.
On that topic, the treads of the tires on the top sides would be supersonic at merely 600kph. Man would these things be loud! They'd basically be giant shockwave generators.