speedometer through a flexible drive cable (Figure 6). This calibration
was determined at the factory by the vehicle’s final-drive axle ratio and
intended tire size and has nothing to do with the speedometer “head”
itself. As a result, speedometer inaccuracies are most commonly caused
by running non-stock size tires on the vehicle. For instance, a small
diameter tire has to rotate faster (make more revolutions) to travel the
same distance as a larger tire. As a result, if you installed smaller-or
larger-diameter tires on your vehicle, its speedometer would be off.
Figure 5
Likewise, installing drive axles with different gear ratios from stock, or
changing the gear ratio of your vehicle’s drive axles, will have the same
effect on speedometer accuracy; a higher gear ratio will make the
speedometer read “slower” than actual road speed, while a lower gear
ratio will make it read “faster.” Installing a different speedometer will
not correct this inaccuracy. It can only be corrected by either changing
the speedometer drive gear ratio in the vehicle’s transmission/transfer
case, or by adding an aftermarket “gearbox” to the speedometer drive
cable.
These little gearboxes are available from many speedometer and