Are you talking about adjusting the the top speed, or shifting gears?
If top speed, you want to set that one or two ticks above the fastest your car can go on that track. The lower the top speed, the better the acceleration.
If you talking about shifting gears, always be in the lowest gear possible with out red lining the engine. The only exception to this is in some RWD cars, you can be 1 or 2 gears higher in sharp turns and that will help keep the back end from slipping out, but then you have to downshift at the exit of the turn to accelerate again.
