How to measure your TRUE tire size!
Hopefully someone will sticky this, for the sake of our collective sanity...
Alot of people ('these days') seem to be measuring their tires and then complaining that they are not true to size... eg. my 35" tires only measure 33.5"
SO, before you start complaining about this, please, please, please measure your tires the right way... this means mounted on your rim, aired up, and UNLOADED (ie. not on your Jeep!).
If you measure the height of your tire when it is on your Jeep and loaded, you can be guaranteed that it will measure small. Why?? Because tire sizes are not specified that way! You have to measure the circumference (and calculate the diameter) with the tire mounted, and aired up, and un-loaded. Yes, good idea is to measure your spare if you can (run a line or tape around the centre of the tire to measure circumference, and then calculate the diameter).
Diamater = circumference / pi
(so take your circumference and divide by 3.14 to get your true diameter)
Measuring from side-side whilst tire is still on the Jeep (and under load) would work OK if you are accurate enough (with where you hold the tape). In fact, many tire manufacturers specify the tire diameter as: "Diameter: The measurement of an unloaded tire from one outer edge to the opposite edge." But not as accurate as the circumference measurement method.
Even if you are looking at section height you have to do it unloaded: "Section Height: The vertical distance measured from the edge of the bead to the center of the crown in a tire that is not under load."
The other way to measure cirumference whilst still on the car is to mark the tire and ground with chalk at the bottom of the tire, and then roll forward until the mark is at the bottom again and mark the ground there - measure the distance between the 2 lines, and divide by 3.14 to get your diameter.
...and just another example of how tire sizes should be measured, from MT:
Overall Diameter (O.D.)
O.D. is the measurement of a tire diameter and is taken from the top of the tire to the bottom of the tire. Note: The O.D. of a tire is measured with the tire off of the vehicle, not under load and at appropriate air pressure. The most accurate way to calculate the OD of a tire is measuring the circumference and dividing by 3.1416”. (Example: Circumference = 88.0” ; therefore 88.0 divided by 3.1416 = 28.01”) Knowing the O.D. of a tire will aid in fitment issues and can assist in gear ratio and speedometer corrections.
All Mickey Thompson® tires are listed in our specification sheets with an O.D. (overall diameter) or circumference. These measurements are derived in the following manner . . .
Please stop measuring tires mounted and loaded, and then complaining that they are not true to size... measure them the right way, then complain your hearts out!
From the land downunder...
MY'13 Cherry Red JKU Rubi.