Originally Posted by biknut
My 80cc has a 36T and will do a little over 35 mph, and I have a 50cc with a 40T and it will do about 30. These are top speeds though. A dependable cruising speed is about 5 mph less.
It's probably a good idea to mention tire size too, because that can make a pretty big difference in gearing too.
My 80 has a 26x1.95 and the 50 has 26x2.125. The 26x125 is a lot fatter and bigger dia than the 26x1.95