Tire Size

[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

[Paw Paw]

OK , JackandJanet. I made it home and found my work sheet that had the figures on it.
WHOO HOO ! JackandJanet, you are exactly right . I found the conversion factor for miles to mm and did the math . I divided the mm per mile by the revolutions per mile and came up with 2483.
I did not know that the revolutions per mile in the chart already had the tire squish calculated in. Thanks for the information .
THANKS AGAIN - JACKANDJANET

PAWPAW

[Jackpine]

You're most welcome! Again though, this is really a starting place (that is pretty close). Your truck may weigh more or less than the weight the tire manufacturer assumes for their table. You may also inflate your tires higher or lower than the cold inflation pressure they assume.

And, even speed has an effect! At 75 mph, the speed limit on the interstates here in AZ, the centrifugal force (and heat) on the tire will cause it to "expand" and increase its "rolling radius" compared to the same tire driven at a more sedate speed of say 55 mph. You can see this "radius expansion" quite easily when a drag racer does a "burnout" to condition his tires. Of course that is an extreme rotational speed, but there will be an effect like this even at lower speeds. This is one reason I don't care for the "measure one revolution of the tire on the driveway" method. I think the result will be low.

So, you are probably within about 2 mph of an "exact" value for speed and distance at 2483mm. If you can do a LONG run like I just did (for 70 miles), you can make it a bit more accurate for YOUR particular weight/speed/pressure setup.

- Jack