Finding the right gearbox/prop combination is tough to do analytically. You need to find a combination that allows the motor to run at its most efficient power level, while giving you adequate thrust and pitch speed for the airplane.

I normally either run the numbers in a calc program like P-Calc at www.flydma.com, or Motocalc, or I just go by what someone else has done in the past.

Off the top of my head, I think a 3:1 gear ratio and a 10x7 prop is a good combination on a Speed 480.