Anyone measure their stock tires, Mine are 30.50" on the jeep, Why I did this was my GPS said one speed and the jeep said another. Checked the tire size calibration and it was set to 30.75". Set the calibration to 30.50" and now both the GPS and the Jeep speed reads the same.. Find it odd they state the 255/75/17's are 32" tall Must have a lot of air in them when they were measured..
I believe 255/75/17's are supposed to be 32.06" tall prior to mounting and having the weigh of your Jeep on them. Once they are mounted on your wheels and have the weight of your Jeep on them they they are only around 30.75" tall.
At least that is how my 255/75/17 BFG Mud-Terrain T/A KMs are anyways.
a good rule of thumb is to take away .75 to 1 inch of what dealer advertises the actual size of their tire..
like jb said.. once tire is at comfortable ride pressures, and weight on top of it.. ur not going to be the same size of the website.. they tend to measure theirs at max pressure with no weight.. makes a big difference.
I wrapped the masking tape around the center of the tread of my spare (unused, fully inflated), overlapping the tape by a few inches. I then cut the tape through where it overlapped to ensure an accurate length. I measured the length of the tape to find the circumference. From there, I used math to get an actual diameter of my tire.
I came up with 99-15/16" or 99.9375" for the circumference of my unused, properly inflated (35psi) spare.
Using the C = 3.14 * d formula, this is what I've come up with (showing my work per my high school geometry teacher ):
99.9375 = 3.14 * d
99.9375 / 3.14 = d
d = 31.8272
So, I've come to the conclusion that my OE tires, when new have a diameter of 31.8".
Now, I know this changes under the weight of the Jeep, and with different sidewall stiffness, so this may be of no use to you regarding the speedo calibration, but thought I'd share what I found.
Are you going to tell him that the speedo and the nav is going to read the same?
Originally Posted by JIMBOX
All tire mfg measure their tires horizontal--
The air pressure/vehicle weight reduces the INSTALLED vertical diameter-
My Hankook LT295/75x16 tire are 33.6" , untill installed, then at 30psi-they're 33.2"
The circumference stays the same, no matter what !
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