The speedo gears are different colors for different rear axle ratios. Transmission websites are your best bet for the gears and yokes.
If you don't know the rear axle ratio, you can find it by jacking up one rear wheel, marking the tire and the driveshaft, and turning the tire around 2 complete turns. See how many times the driveshaft has turned as you spin the wheel (tire). If the axle has turned 3 times, the ratio is 3:1 (3.08:1 rear end). If the axle has turned about 3 and 1/2 turns, it's a 3.42 rear end, etc.