Use the 500 mph cheat. Go to user name and type in SPOILT. Then change name to XCNTRY. Start a single race. Use the hidden "AutoCross" track with the El Nino car. After the two tunnels you'll find the wall on the left goes straight up.
Drive up the wall until the car is falling. This will get you thousands of miles per hour. Check your speed in the race summary. I once got 6453 mph.