Runtimes, and curious about LiPos...

[lipomax]

When figuring out speed, you should really take into account the voltage drop. For a good set of batteries, you can expect a 1.2 volt cell to drop down to 1.1 or 1.05 or 1.0 volts if you really jack up the pinion gear size. If you figure that, your speed probably drops to below 40 mph with 18T pinion and probably closer to 40 mph with 20 T pinion.