The Michelin Truck Tire book lists the radius of a loaded XZE 255/80 at 17.9 inches and the radius of the 255/70 at 17.2 inches.
The 255/80 rotates 538 times per mile, and the 255/70 563 times per mile.
So 65 times 563/538 equals 68. That explains most of what you are seeing.
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Richard Rhonda Ty and Alex Entrekin
1995 Newell # 390 DD Series 60, Allison World Trans
Subaru Outback toad
CoMotion Tandem
Often wrong, but seldom in doubt
Rhonda's chronicle https://wersquared.wordpress.com/
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