Way back in the dark ages, there were speed change gearboxes that were used to correct speedometer errors. They were attached to the transmission and spun the speedo cable at a different speed, higher or lower. Internal gearing in the small rectangular box could be swapped to get the right speed correction.
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!