Gear Ratio Analysis Program...
You can use this to determine gearing.

Rotax Max 125cc	Engine Gear Axle Gear Circ = D x 3,1416 RPM meter
Engine Gear teeth   Axle Gear teeth   Gear Ratio       Tire Circumference inches mm Maximum RPM rpm   Top Speed     mph km/h

Short Track

	78	79	80	81	82	83	84	85
12	6.50	6.58	6.67	6.75	6.83	6.92	7.00	7.08
13	6.00	6.08	6.15	6.23	6.31	6.39	6.46	6.54
14	5.57	5.64	5.71	5.77	5.86	5.93	6.00	6.07

Long Track