I'm guessing that it is a combination of measurment error, factory speedo calibration, and tire brand/inflation/wear.

The calculated diameter for a 265/75R16 tire is:

16" + ((2 x (265 x .75)) / 25.4) = 31.65"

Today I measured my tire circumference by chalking the bottom side of the tire where it met the ground and the ground and then pulling forward for three revolutions of the tire. That was 287.5".

Divided by three to get the average circumference and then again by Pi to get the diameter. The result was 30.505".

My tires are the factory BFGs inflated to 60 PSI and have 26,000 miles on them. I can tell that they are fairly worn from factory new, probably about 0.75" - but certainly not 1.145" worth. This tells me that the 31.65" from above is the ideal unloaded diameter and that the rolling diameter is something less.

So with the tires worn as they are now, when I'm doing 70 MPH my RPM will be 2048.471. When the tires were new and the extra 3/8 inch of tread (3/4 inch more diameter), the revs for 70 MPH would have been 1999.316 RPM. A difference of 49.155 RPM due to tire wear alone.

Just thought I'd toss this in as interesting info.

Daryl

 

No one says it like Daryl...........................