Re: A governor, or speed limiter,
I have no idea to help you with. Why not just give it less gas at the throttle? What kind of motor are you using? I'm just curious why you want to go slower. It is kind of refreshing in a way, since almost everybody here wants to go faster. For myself I'm pretty happy going 20-25mph and approaching an incline to goose it to 30 to help make it up to the top without peddling. If I couldn't give it full throttle I wouldn't make it up the hills around here.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...