Tire Size

[MileHigh04SCREW]

Another way, personal favorite lazy-way, is to chalk a tire on the edge near the tire tread and on the ground, and then roll one complete tire rotation (making sure your tire chalk mark is exactly where it was before the rotation - a plum bob works well). Then straight-line measure the distance. Just make sure all of your tires are air'd up correctly and evenly.

milehigh

[Paw Paw]

Hey guys ! My actual calculation on tire size came up to 2559 - not subtracting anything for tire squish. So, I decided to set the tire size to 2550 as a starting point.
Did the Longshot270 suggestion an got a milage distance from Mapquest on a route that I was going to travel on Sunday evening . Zeroed the trip odometer on the truck .
Mapquest mildage 11.03
Truck 10.9
11.03/10.8=1.011926
2550x1.011926 = 2580
This is where I am right now . Should I leave it at 2550 or bump it up to 2580?
I grew up working on the ole FLATHEAD V8'S , and 8N Ford tractors (which I still have ) and this whole digital thing is just too much fun and facinating .
Gotta love it .
THANKS FOR ALL THE REPLIES
PAW PAW

[Jackpine]

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

[Longshot270]

I know, aren't these numbers fun?

I used pegasus and inputted the same amount of squish as my truck and I got about 2380. Your tires are slightly larger than mine (235/70-17) and mine come pretty close to 2332. I think that number is about 1/10 of a mile off at 25 miles.

FYI, I used about 5% tire squish on pegasus. That program is set up a bit different compared to the hand calculations.

I have also had issues with mapquest, google has been right on target when I zoom in and manually pick my two points and the route to them. If you type in two addresses there is sometimes error from the points not matching up to the gps they used to map it.

[Paw Paw]

Thanks JackandJanet . I have Michelin LTX M/S 2 tires . Tire Rack says the diameter is 32.1 " and Revs per mile is 648 .
Thanks for all the help.
PAW PAW

[Jackpine]

Great! You found the page I was referring to. The 32.1" diameter is with no weight on the tire (and you need this value to check for clearance at the top of the wheel well or at the sides when the wheels are cornering). The revs/mile figure IS with weight on the tire and works out to a TS of 2482mm. I think that is going to be VERY close to the number that would be dead accurate.

- Jack

[Paw Paw]

Thanks JackandJanet . I am at work right now and do not have my figures in front of me . I will check my numbers when I get home and get back with you .
THANKS AGAIN FOR ALL THE HELP !!!!
Paw Paw