Paw Paw, I don't trust the mapquest mileage in your case. If you DON'T factor in "squish", your PCM's going to record a LONGER distance than you actually traveled.
No matter what Tire Size you use, the tires rotate a "fixed amount" in a fixed distance. I think mine rotate 647 turns in a mile. So, in 11.3 miles, they turn 7311.1 times. With a TS of 2485, I think the PCM will "see" 11.3 miles for that many rotations. But, if the entered TS is bigger, at 2550, you get:
(2550 x 7311.1)/25.4 = 733988.39 inches, or 733988.39/12 = 61165.7 ft and that is 11.58 miles on the odometer.
By the way, you have the same size tires as I have (275/65-18). I am now using 2485mm for TS in the old Gryphon or a 31.1" diameter in the CTS, at 40 psi inflation.
Tire rack says my Goodyear Wrangler SilentArmor tires spin 651 revs per mile. They expect more "squish" than I experience, possibly because they assume a lower inflation. If they were a particular Michelin, they say 648 revs. A Yokohama tire is 645. If you tell me the exact make and model of your tires, I can point you to the page that will show you the manufacturer's "rev" number, and from that, it's an easy calculation to TS.
But, I am CERTAIN that 2550 is TOO LARGE!
- Jack