Proper measurement of a tire is very important when choosing a tire as it will affect many things such as engine R.P.M., vehicle fitment, roll out and ground clearance issues.
Roll out is the number of inches the tire rolls before it has traveled back to its initial starting point as shown below.
A tire can be measured for circumference or roll out by using a tape measure. Start by laying the mounted and inflated tire on its side. Wrap the tape measure around the tire going around the center part of the tread pattern.
Our bias race tires (ET Street R and ET Drags) are made with a nylon construction. For this reason, it is necessary to use the following procedure for checking the rollout (circumference) of tires when they are first mounted. Always check the size before installing rim screws or putting the tires in use.
Before mounting the tires, we recommend checking the date codes (Refer to General Tech Bulletin # 1 (LINK)). A natural characteristic of nylon is to shrink over an extended period of time. The dates should be within six months to help ensure close measurements. If they do not, please contact your dealer or point of purchase.
After mounting the tires, take the air pressure to 20 psi then immediately drop to the pressure you plan to use at the track (example – 12 PSI). Important, never let one tire sit at a higher air pressure for an extended period of time. To clarify, do not air one tire up, drop it immediately, then let the other tire sit at 20 psi for while. This will give inaccurate numbers as one will have stretched. Before the tires are run they should always have the same air pressure for the same amount of time unless you are trying to stretch it.
Tires must be matched within ½” maximum before they are run. If they exceed this difference, it can usually be corrected. This is done by over inflating the shorter tire and letting it sit a short period of time to allow it to stretch. It is very important to monitor this procedure to avoid stretching too much. When stretching tires always drop them back to “0” psi,, then back up to your track pressure. This will show if it has actually stretched or not. This should be done as close to running the tires as possible. Never inflate tires over 40 PSI to stretch them. If you cannot get the tires to match, please contact your dealer. Mismatched tires exceeding ½” cannot be exchanged if:
- This procedure has not been followed.
- The tires have been put into use.
- Rim screws have been installed.
- Again, this procedure will only work before a tire has been used.
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 . . .
- M/T® Slicks:
-- All tires taken to 20 PSI then:
-- Measuring rim at 10 psi cold.
-- (Exception: 33″ drag @ 6 psi)