I run a modified GT5R 66cc "Super Rat" china girl engine, shaved stock slant head for a little more compression, a re jetted carb, a hotter ignition, a SPB Expansion pipe and shift kit, a Pirate centrifugal clutch.
(this clutches spinning mass gives the china girl some "fly wheel" effect, its noticeable)
This all feeds through a shimano 11/32-9 speed rear cassette with a 48tooth driven chain ring and a 36 tooth drive chain ring.
The stock clutch is only used to bump start the engine, once running its hands off the stock clutch.
This is a good set up if you like to peddle because that big 48 tooth can really help out the engine.
I always start off in 1st or 2nd gear, peddle it up to like 10mph before I get on the throttle. This also relives undue shock on the bikes derailleur drive train.
Once under way I up shift and peddle, I listen to the engine, I can hear and feel the the sweet spot at about 35 to 40 mph .
Top speed is an observed 57 mph on level ground, it pulls over a 350lb load with me, the weight of the bike its self, my gear and a half gallon of gas, no wind, 2 way average, but i try not to push it that hard.
So its barely breathing hard at 35 mph.
I know im taking strain off the engine by keeping the RPM down, helping it live longer and getting way better gas mileage.
Yes... it could easily pull my fat behind away from a dead stop, but why strain the engine and drive train?
Peddling has also helped me drop 20 lbs so far, down from my usual winter 270lbs... My doctors are congratulating me, my blood pressure is down, and I'm having a ball NOT being stuck on the subway coming home from work at 4 in the morning.
The ride home over the Manhattan bridge to Brooklyn is a treat at that hour, less traffic, fewer cops and you can really get on it!
Theirs a reason these things still have a crank and peddles...ITS STILL A BICYCLE... peddle it!
Have fun, and watch out for Kamikaze cab drivers!