Here is a wheel diameter to speed/RPM chart to make the first step a little easier...
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=16114
Once you decide that you will be using X volts and Y amps, that results in a certain amount of watts. If 1,200W (as a common example) is used on a system that is geared to run at 50-MPH, it will have very weak acceleration and hill-climbing. It will only achieve near 50-MPH on flat land with pedaling added. If that same 1,200W is geared for 12-MPH, it will be slow (of course), but it will have exceptional hill-climbing, with very few heat issues.