It takes power to make a moped move, and more power to make it move faster. A small engine has a very narrow powerband (the rpm range where it makes the most power). Thats where a transmission comes in, so you can keep the engine in it's powerband at different speeds. But you can still only go so fast, no matter how many speeds you have, because you run out of power. A vehicle, moped or otherwise, is constantly accelerating, just to overcome wind resistance and rolling friction, but only to a point. Get on your bicycle, and see how fast you can go. Higher gearing will help to a point, but eventually you simply run out of power. A manual transmission would be a great addition to a moped, but it would have to be an internal motorcycle type transmission. Bicycle transmissions will not work with the speed and power of even a stock moped. I have destroyed several derailleurs on my mountain bikes, with no motor. Even a top of the line Shimano XTR. Peace. Jerry.