There is a limit at around 200 m/s (720 km/h) where the game stops you from going any faster. Don't remember the actual reason for it, something relevant to either safety, hacks (in pre-mod era this speed was simply not possible to achieve naturally) or maybe the physics code stops working properly at so high speeds (after all at that speed you are getting out of the general subsonic aerodynamics territory and the code might not work at all nearing the transsonic speeds).