Well, I'm afraid this is a bit vague.
Dragster is basically a straight line race. So using the bullet behavior for your object will allow you to move it in a straight trajectory.
As for shifting mechanic, I guess there are several ways of doing it.
As I can remember in the drag races of Need for speed underground for example, you have a counter that displays the engine's RPM. And when the engine's is in a certain range of RPM you press a key to gear up, which ends up lowering the current RPM but also changes the next "gear up" range and the current "max RPM" that can be reached.
Would that be what you are looking for ?
If not, then you have to break down the mechanic of what should happen, this will make it easier to determine how to implement it.