[CapriRacer]

Originally Posted by Jim Allen

........... and I measured the tire circumference to the millimeter at my standard pressure, entered that into the programmer ............

A word of caution: The rolling circumference of a pneumatic tire is about 3% less than the measured circumference.

Originally Posted by Jim Allen

........... and my speedo is deadnutz. .........

which may mean the speedometer is calibrated to actual circumference and internally compensates for that 3%. Other vehicles may not do that!

Originally Posted by Jim Allen

............. As the tires wear, I can remeasure to keep the speedo as accurate as possible. On my other cars, I determine a correction factor as accurately as possible and use that in calculating my true miles traveled.

Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

If you look closely at what Tire Rack did, they took a tire with 12/32nds tread depth and compared that to a tire with 2/32nds. That would be 10/32nds difference in radius = 20/32nds in diameter = 0.63" on a diameter of 24.9" or 2.5%. They only got a 1.5% difference in distance traveled!!!

Tread depth differences don't directly translate into circumference differences!

Also, many of the revolutions per mile (rpm) values are CALCULATED values. And take it from someone who used to do those calculations, the formulae may be old and unverified for many, many years, so their accuracy is suspect. Besides, most folks who would do those calculations would think that within 2% is close enough!!

Also, if you do the calculation on the values that Tire Rack has in the artcle, you'll see the diameter values vary from the rpm by 2½ to 3½%.

So use caution when using published rpm and dimensional values!