The maximum speed the trucks cap off at when driving in reverse amounts to 12.3 undecillion miles per hour (for perspective, this number is 1.8341329e+28 times the speed of light), because no upper limit was implemented by the developers to cap off the speed for driving in reverse at a more realistic limit. After this speed is reached, the speedometer displays a value of -001.$ and causes you to win the race despite having driven in reverse practically infinitely. This is because as a truck travels faster in reverse, it will gradually travel along more of the surface area of the race track until it covers every pixel on the map simultaneously including all of the race track's checkpoints, meaning that the truck won the race because it was everywhere at once.
Four unused cars can be found in the game's files. They consist of a Ford Expedition, a Toyota Celica, a Hummer H1, and a Nissan 350Z. The Ford, Celica, and Z350 are fully functional, but the Hummer's materials file doesn't point correctly at its model and textures, and only draws the alpha channel on most of the vehicle.