Figure the diameter of the rear tires, multiply times 3.1416, then divide that into 63,360 to get the revs per mile. Then multiply the gear ratio times 7(number of speedo drive teeth in most Ford transmissions), multiply that result by the revs permile, then divide by 1000 to get the speedo cable gear(tooth count) you need to make it right.
The white gear doesnt last very long. I think it says 12000 miles if I remember right. The speedo works great with it though. I thought it might be bouncy.