The X2 method is definitely NOT the best way to measure tire size. The total tire height method using a bubble level mentioned by Turbo21 is better, and the roll out method suggested by Fiend is the most accurate. If you don't believe me, keep reading. BTW, Eddie's method referenced earlier is not intended for use in a programmer tool, but rather to establish a database of tire sizes that can be compared against each other.
The vehicle uses tire diameter to calculate circumference, and then revolutions per mile. Tires are not round, because they squat under load, so a NOMINAL diameter is required. The X2 and bubble level methods are only approximations for this nominal diameter. Even when squatting, however, the tire circumference does not change, so the most accurate method to obtain the nominal tire diameter is by directly measuring the circumference under load and calculating the nominal diameter. (This is a bit like approximating the circumference of the tracks on a battle tank using a circle with nominal diameter.) I tried this on my rig using painters tape and a tape measure and compared to the other methods on the same tires to show the differences. The table below summarizes the results. Notice that the bubble level method gives half the error as the X2 method, and the nominal tire diameters for the X2 method are underestimated by as much as 1.70 - 1.90".
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The jeep weighs 5500 lbs at the time of measurement, and is running stock BFG Mud Terrain T/A P255/75/R17 tires with 60,000 miles on them. Measurements were done at an elevation of 100 feet at 90F.
