21.5 FDR
To be honest, I am currently building a 21.5 car...can some other guys chime in on what they are running please? I am running a black can tamiya motor...BUT I think a good starting point is 4.5 FDR, I think some guys are running 4.0, but they are running cooling fans, etc. Also, it depends on your mechanical timing on the motor. I would bring some gears a few teeth above and below 4.5 and go from there.
Sorry, I don't know the exact answer, once I get my car running I will have a better idea, but it is still in 300 pieces on my desk at work...lol...
Coach