Use larger wheels.
This is probably your best bet. Im using the same principle on a spring powered vehicle in my engineering class in school and so far, my vehicle is going further than everyone elses because ive got bigger wheels. It really does make a difference.
Dont put 48 volts to your servo and expect it to reach mach 3
a 6 volt rated servo should get a MAX of 7 volts... it might take 8 volts but only for a few moments before it melts.