Originally Posted by
bobslandies
Easier than that. If you have an older local auto instrument technician he will have a cable turn meter. You disconnect the speedo, plug in the cable turn meter and drive up and back a number of times over his measured mile/kilometre.
Have someone else taking the readings and average them. This will give the number of turns and he can change the gears inside the speedometer head. I had a speedo converted from 4.7: to 3.5:1 (Ford 9") in my Series in 1975.
Older techs were doing it all the time for taxis, local Highway Patrol cars, etc.
Today its mostly restored bike speedometers they rebuild that get this treatment.
Bob