"Effective" Tire Diameter
By Bowling

The diameter of a tire which is determined by the tire size imprinted on the sidewall:

Diam = 2 * (((SectionWidth * profile)/(100 * 25.4)) + (wheeldiameter/2))
(e.g. For tire 205/70R15, SectionWidth=205, profile=70, wheeldiameter=15)

is not the actual "effective" diameter, because the tire is not a perfect circle. Due to vehicle weight, the tire flattens to the road which reduces the real wheel radius. This effect, although small, really adds up when doing calculations requiring rolling tire diameters to determine distance, etc. One can attempt to measure the effective radius by estimating the center of the hub and using a ruler, but this is sometimes difficult. There is a more accurate method:

Measure the diameter of the tire (tread-to-tread) PARALLEL to the ground (i.e. at 3:00 and 9:00 positions on the wheel, thinking of a clock) and note this as HDIAM, in units of inches. Now measure the diameter from the ground to the top of the tire (from 6:00 to high noon) and note this as VDIAM, in units of inches. VDIAM had better be a smaller number than HDIAM, or there is something wrong, since VDIAM is the flattened diameter. Since it is easy to do, get the tread width size in order to compute contact patch area. Easiest number to use is the SectionWidth value, which comes from the sidewall (in mm, see above), or you can measure it. Plug-n-Chug

Many Thanks to Mr. Bruce Bowling

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