If my math is right and the speedometer is dead-on at 1000 revolutions per mile, that wants a 6 & 19 combination. 7:21 is only about 5% off and not enough to worry about in most cases. About 63 mph actual vs 60 mph indicated. Tire wear from stickers to wear bars represents a speedometer calibration change of half that much (~2.5%) and nobody except time-speed-distance rallyists ever worried about that.

You used to be able to get external correction gearboxes for fine speedometer/odometer calibration, but I don"t know about now. IIRC, it was Halda (a European outfit) that supplied the TSD rallyists back in the day. I also know for a fact that GM used the same sort of thing in some of their late 70"s cars.


Norm