To superjump/gain a lot of speed really quickly, press F3 on your keyboard just before a collision with a preferably tall, straight flat wall. The game will technically pause but your car is still moving in the background, so press F3 again when you assume your car has hit the wall to "unpause" the game. This will send your car flying forwards.

This works best just before jumps or any airtime, on a flat wall. The jump out of the tunnel on Times Square is a good example.

This doesn't work very well on curved walls along flat roads, like the entirety of Race Wars.