I'll make a quick explanation. I used a method called "snaking". Some people consider it cheating, some people consider it part of the game because you don't put in a code or anything... either way, I'm going to explain how to do it, and why it works. First off, you need a heavy vehicle, with a high grip rating. Black Bull is my favorite, and most others too for this trick. Set the meter all the way to acceleration, and then start the race. Get a little bit of speed (not much is necessary) and then proceed to do the following.
Hit L+Left, and then R+Right, and alternate this. Be careful, it's very difficult to control the vehicle this way, as you're almost ALWAYS going out of control. The physics engine will be tricked, as it will think you're constantly coming out of a sharp turn drifting, and it will give you a speed bonus. You keep stacking them on and you will gain a tremendous amount of speed. You can't do this on every track. For example, doing this on the first track of diamond cup is almost impossible, no walls and very narrow. I've gotten Black Bull to 2700 km/h without boosting on Sand Ocean Surface Slide. Cheating? Part of the game? Whatever it is, it gets me tickets for beating staff ghosts. Good luck to ya guys, and if ya got any questions, lemme know.