The method behind the numbers

Inputs and definitions

For a metric radial tire marking, W is nominal width in millimeters, A is aspect ratio in percent, and R is nominal rim diameter in inches. The model does not use load index, speed rating, tread depth or construction details beyond the recognized radial-size notation. A published marking is a label, not a measurement of the tire on your vehicle.

Tire geometry

Sidewall height H = W × A ÷ 100. Rim diameter M = R × 25.4. Overall diameter D = M + 2H. Nominal circular circumference C = πD. Revolutions per kilometer = 1,000,000 ÷ C. Revolutions per mile = 1,609,344 ÷ C. All lengths in those formulas are millimeters. The circumference model treats the tire as a circle; it does not model its loaded deformation.

Difference and percentage

An absolute difference is new − original. A percentage difference is (new − original) ÷ original × 100. Because circumference is π times diameter in this model, their percentage changes are equal. A revolution count changes in the opposite direction. Swap the inputs and recalculate; do not merely change the sign of a rounded percentage.

Theoretical speed comparison

Theoretical speed = indicated speed × new circumference ÷ original circumference. This isolates the size-ratio effect under an assumption that the original calibration corresponds to the original nominal circumference. It does not measure an instrument’s existing bias or a tire’s real rolling circumference. Displayed speed and theoretical speed must use the same unit in a row.

Wheel position and sign convention

Wheel width W is converted from inches to millimeters. Inward distance I = W ÷ 2 + ET. Outward distance O = W ÷ 2 − ET. A decrease in I means less inward reach from the fixed mounting plane; TireSpan describes its opposite as a gain in nominal inner space. An increase in O means farther outward. For an outward-positive coordinate drawing, the inner position is −I and the outer position is O. These are rim-width reference planes, not measured tire or rim-lip boundaries.

Units and rounding

One inch is exactly 25.4 mm. One international mile is exactly 1.609344 km. Calculations retain full numeric precision until display. Changing a unit does not re-enter a rounded result as a new input. Display rounding removes negative zero. Downloaded machine values use a decimal point and explicit units; a localized display may group digits or use a different decimal separator.

Check the reference example

215/55R17 gives H = 118.25 mm and D = 668.3 mm. 225/45R18 gives H = 101.25 mm and D = 659.7 mm. New − original diameter is −8.6 mm, or −1.286847…%. At an indicated 100 km/h, the theoretical value is 98.713152… km/h. Reversing the pair gives +1.303623…% and 101.303623… km/h.

How directory sources are used

The directory records a specific manufacturer source for each marking. Product prefixes or ZR notation remain in the source record even when the page uses a normalized geometric form. This normalization does not erase differences in load designation, speed capability or intended use. Public comparison questions support editorial selection only. TireSpan does not reproduce a fitment recommendation from those discussions.

What the model leaves out

No result certifies fitment or safety. The model excludes load and speed ratings, approved rim-width ranges, wheel construction, brake and suspension clearance, bodywork, inflation pressure, wear, deformation, calibration and local requirements. There is no automatic “within three percent is safe” rule. For a replacement decision, use the relevant vehicle and product specifications and competent physical assessment.

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