Kendall -

Some time back, anything within plus/minus about 10% was considered "within tolerance" for speedometers, although most new cars "delivered" with 5% or better when new.

The gear where the cable came out of the prop shaft usually was available with around 17 to 23 teeth on it, odd numbers only, and you could only add or drop two teeth at a time, so the only adjustment that could reasonably be made was about 17/19 = .89, 19/19 = 1.00, 21/19 = 1.11, if you started at the 19 that was common.

In the early 60s, US automakers sized the tires to get an average of 10,000 miles on the tread. This was considered a minimum that buyers would tolerate, and let them use the smallest tire "almost big enough" to improve "ride" and to save about $3.00 per tire. (No comment on which was most important.)

(The use of undersized and overloaded tires, invariably bias-ply, also contributed somewhat to the sales pitch that "radials are better" and helped drive the switchover to (almost) universal use of radials. Coincidentally(?), radials are cheaper to make than bias ply ones, once the manufacturing equipment switchover is made.)

My '64 Buick Special came with 6.70x15 tires, and they were pretty well shot at 10,000 miles - so I replaced the set at about 14,000. In the replacement I went up to a 7.10x15, still bias ply, sized to carry the load with "flat wear" on the tread - the "proper size for my vehicle" according to the Tire & Rim Mfrs Assn Handbook. That set got well over 38,000 miles and were replaced mainly because of "sidewall rot" (almost 6 years later) rather than tread wear.

The tire size change unfortunately put my speedometer "quite a bit out of spec" so I had to drop two teeth in the gear to bring it back to within a little less than 3%.

Tire changes, and poor pressure maintenance, can easily affect speedo calibration enough to account for the difference seen; and, especially for older cars, sometimes they just weren't really all that close to reality to start with.

John