Tire size also is a big part of it. I always ran around a 2.5 tires on my Bracket cars , With gearing around 21 / 90 and just go from there ! But this should be a good starting point. also depending if the motor is a single , double ,triple turn ECT also matters. I always use the Final drive for a Good referance. the gear I gaver gave you above = a 4.28 . ( 90/21=4.28 ) so anywhere from about a 3.70ish to 4.50ish depending on tire size.