I have a Huasheng 49cc with 5:1 4G on a shift kit, 3-speed rear hub, top speed is 39MPH @ 7800RPM.
No real engine mods yet, it's got an SBP airfilter and about 34" of flexpipe with a 5-7 horse Briggs muffler.
My cruise speed can be anywhere from 6MPH to 34MPH, it's a very flexible setup.
Edit: When I had 2-strokes, my top speeds were 28 with 44T, 35 with 39T, and 42 with 34T.
Cruise speed was 15-25 with 44T, 15-30 with 39T and 20-34 with 34T. The smaller gears make for a very quiet bike under 30MPH.
Going any faster and I prolly would be wearing a piece of chinagirl in my lower extremities to this day!