Tire size-speedometer calibration

[Norm & Judy]

On a recent trip, I took along my Pioneer GPS unit. I noticed that the indicated speed on the GPS was at 65 when the coach speedometer was at 70.

The Data tag shows original tire size in as 255x80x22.5. The coach now has the lower profile 255x70x22.5 tires.

My question is could this possibly account for that much difference in the speedometer readings?

[The Newell]

Yes, it could. Assuming 9" rims, roughly, the 80 tires would have a 7.2" depth while the 70s would be 6.3".

Adding each to the rim radius (11.25") results in 18.45" for the 80s and 17.55" for the 70s. The difference is 0.9", about 4.9%, which would result in a speedometer reading about 3.2 mph high. Not all the difference, but obviously significant.

[Richard and Rhonda]

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.