Can my speedometer be corrected for accuracy? My odometer does not work, and I will fix that, but according to my gps it is fast. Now I previously thought the GPS would be innacurate, but is pretty close with the ML speedometer. I would like to keep it as accurate as I can. Oh this is an 86 420sel by the way. Thanks

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual