Get a speedo app for your phone. Drive 50 MPH by gps, the dash speedometer reading either + or - X2 will give you the ratio you need , then find a ratio adapter to speed up or slow down the speedo cable by that much.
Interesting solution. So if I drive 50 based on my phone GPS, and my truck speedo says 60 . . . Then I should multiply 60 X 2 and would get 120? Or should I multiply the error of 10mph X 2? That seems more logical since 10 X 2 = 20 and the MPH is 20% off. Then I’d need to order a speedo reduction adapter for 20%.
