Easy. Jack the car up and put it up on stands, and put car in neutral. Have a friend count how many revolutions of the drive shaft it takes to rotate the tire one time around, that is your gear ratio. Approx 3 3/4 turns of the drive shaft to one revolution of the tire is 3.73 etc. After that get the proper speedo gear for that gear ratio. The speedo gears are different colors depending on what gear ratio. Do a search, Im sure there is a chart here that could help you.
