In these formulas, the variable item, to me, is "Tire Diameter", usually listed in "Inches". I know that many people talk in tire sizes that way, for ease of use, usually. But this is where the "Revs/Mile" figure comes in, which is better terminology for the same, eventual thing.
In the tire spec sheets, for particular tires brands and sizes at
www.TireRack.com, both "Revs/Mile" and "Diameter" are listed. Quite easy to find! To me, "Revs/Mile" is the more accurate measure to use.
In the yearly "Hot Rod Annual", possibly #1 in about 1962, there was a simple formula to determine road speed for particular rear axle ratios and tire sizes. "Tire size" was determined by chalk-marking the tire's tread area, then slowly moving the car until you had two chalk marks of the pavement. Then measure the distance between them and that was what came to be "Revs/Mile", after that distance was converted into "Feet" and then divided into "5280 feet/mile". There were also formulas to determine torque from horsepower, and other car-related things, too. I used lots of paper and ink to do those things, back then! The charts in the provided link short-cut that somewhat.
In the A.M.A. Specifications (which all OEMs had to submit to that group for each model they built, back then), the "Revs/Mile" was listed at a particualr road speed. 30mph in earlier years and a bit higher in later years. Using that road speed accounted for a speed before the bias-ply tires' diameter would not be affected by centrifugal force, enlarging the diameter a bit. The later bias-belted and radial tires would decrease such, due to their outer belts keeping the sizing more constant.
At one time, somebody noted that using the chalk mark method to determine tire size took into account the compression and expansion characteristics of tires, which might vary from just a simple tape measure around an unloaded tire's diameter. The formulas would like for the latter method to be accurate, I suspect.
Also note the "Converter Slip" factor in some of the charts. An automatic transmission torque converter always has a small bit of slip in it. Although I have always felt it would be less with the 3-spd TorqueFlites than for other brands of vehicles. In a time well before "lock-up" torque converters were in production.
Several thoughts,
CBODY67