Many older vehicle owners eventually have to decide whether to continue using hard-to-find or increasingly expensive bias-ply tires in their collector cars. The old bias-ply tire sizes were typically much taller (skinnier) than modern low-profile tires, which makes them practically impossible to directly upgrade. While it is possible to try to find a similarly sized radial tire, the upgraded radial tire size isn't always exact and is often undersized, which in turn affects the accuracy of the speedometer and odometer.
Rather than looking up a radial equivalent of a old bias-ply size, it is more accurate to determine the tire size that will keep your odometer (and likely speedometer) accurate. The odometer is simply a counter that relates tire rotations to distance traveled. The speedometer is simply a spring-loaded pointer that relates the speedometer cable's rotational speed to road speed. Therefore, to determine the correct tire size, we need to work backwards from the odometer. Generally, 1000 revolutions of the speedometer cable record exactly 1 mile and, obviously, 1/10th of mile requires 100 speedometer cable revolutions.
The speedometer cable is driven off the output shaft of transmission by a speedometer pinion gear. The factory had a selection of speedometer pinion gear sizes to compensate for the variety of available tire sizes and axle ratios. The speedometer gear ratio is simply the ratio of the number of teeth on pinion gear compared with the number of teeth on the output shaft. If the factory installed a larger tire, they would use a pinion gear with less teeth. Conversely, if they kept the same tire size but installed a low ratio (high numerically) axle, they would use a pinion gear with more teeth.
At the axle, the differential increases the torque produced by the engine to the rear wheels. Each turn of an axle shaft causes the tire to turn one revolution and the distance travelled in one tire revolution is related to the diameter of the tire.
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