If anyone is running the 255/70/18 tires like I am and has their ultragauge set up correctly for mph and distance travelled, would you care to share the number you are using? The math says the 255/70 tire is 100.7" circumference and the 255/55 is 91.2", so i assume I should just put in a 1.104 factor, unless the ultragauge uses a percentage, and then it would be about 90.6%. I've read the write-up, but I'm about 30 miles from the nearest road with mile markers, so I'm trying to cheat a bit and get 'er close. Thanks in advance.

 

I used this site:
Tire Data Calculator

From this site, the Tire Rolling Circumference calculation shows:
255/70R18 = 2471.1 mm
255/55R18 = 2248.5 mm

The offset factor is 2471.1 / 2248.5 = 1.0992