The first thing to do is establish what percentage of error there is between indicated speed and actual speed. Time a measured mile, and jot down the indicated speed. 60 MPH indiccated makes a good reference point in most cases. Let's say you run a mile at an indicated 60 MPH, and it takes 50 seconds. 60 divided by 50 equals 1.2, indicating 1.2 miles per minute, or 72 miles per hour. That's a 17% error at that speed, so you would need to speed up the speedometer cable by that much. I doubt if internal gears are still available to make the correction inside the transmission, so you'd need an external gearbox to drop the cable speed by that amount. Simply figuring the amount of error at a specific speed will give the vendor of the correction box a reference point to start with. If you want the correction factor at a lower speed, driving a mile at 30 MPH should take 2 minutes, etc.
Jerry
