I use a Garmin GPS for a speedometer since I changed the tire size on my road trip truck- - - -resulting in a better cruise RPM with less toggling in and out of torque converter lockup constantly. That gave me about a 5% speedometer error, indicating faster than actual road speed MPH on the speedo. There are correction gearboxes made to alter speedometer cable rotation speed to account for different tire sizes, 2-speed rear axles, etc. Also, some transmissions can be fitted with different-size speedo drive gears, using an offset-center gear carrier that can be rotated to accommodate speedo cable drive gears with different numbers of teeth.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!