Hi,

A good 50cc engine turns a 22x8 at about 7000-7500rpm with a bolt-on muffler. When using a tuned quiet pipe the power output and torque becomes much higher so the engine can turn a 22x10 or 23x8 at the same rpm. A good guess is that a 22x12 will turn about 6500rpm but make sure the pipe header is long for smooth running behaviour.