The simplest way to figure speedo error is a timed mile. Find a road with mile markers, and drive various speeds until you make a mile in exactly 60 seconds. That's 60 MPH. Figure the percentage error between a 60-second timed mile and the indicated speed on your speedometer, and either swap speedo drive gears in the transmission output housing or have a speedometer shop build up a speed-change box for you.

Here's an example:

Timed mile = 55 seconds
Divide 55 into 60 = 1.09
60 X 1.09 = 65.4 MPH

If your speedometer reads above or below 65 MPH, you need to correct the gear ratio of the speedo drive.
Jerry

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