This depends on the diameter of your tires.What should I think about for gearing though
Lets just assume your tires are 24" diameter and your motors max RPM is 6000 and you want to go 55 mph at that max motor RPM point.
A 24" diameter tire will advance 75.3982 inches per revolution (diameter * pi.) 55 miles per hour is 3484800 inches per hour (55 * 5280 * 12.) 3484800/75.3982 inches gives 46219 revolutions per hour. Divide by 60 to get an rpm of 770.3. To get from 6000 motor rpm to 770.3 wheel rpm means you need a reduction ratio of 7.79 : 1 (6000/770.3).
You are probably going to want to set the 55 mph point at a little lower motor RPM. This would be to keep the motor operating within its torque curve.