To figure out correct speedo driven gear please use the following formula:

Driven Gear Teeth =  Drive Gear Teeth x Rear Axle Ratio x Tire Rev. Per Mile
                                                                   Constant

Example of a Customer having 3.73 Ratio Ring & Pinion, with a Manual Transmission featuring a 7T Speedo Drive Gear would be:

7 x 3.73 x 815 = 21.27 or 21T Red
        1000

Tire Rev. Per Mile & Speedo Drive Gear Teeth Count may change depending on the application.  Feel free to call in if not sure of any of the information needed to figure out the above Formula.