Tire Size Comparison Calculator

Compare two size markings

1. Enter the reference and alternative metric radial sizes without load or speed-rating suffixes.
2. Choose metric or inch output. Width in the marking remains millimeters and rim diameter remains inches.
3. Read the nominal dimensions and qualified speed example, then check vehicle and manufacturer requirements separately.

From size marking to nominal geometry

In 195/65R15, 195 is nominal section width in millimeters, 65 is sidewall height as a percentage of width, R means radial construction and 15 is rim diameter in inches.

Sidewall height is width times aspect ratio divided by 100. Add two sidewalls to the rim diameter, using 25.4 mm per inch. Multiply diameter by pi for geometric circumference.

Percentage change uses the reference diameter as its denominator. Swapping sizes reverses the difference, but generally changes the percentage magnitude.

The speed example assumes an initially accurate instrument calibrated to the reference, unchanged calibration and rolling circumference proportional to nominal diameter. Real rolling dimensions and instrument behavior can differ.

Examples with explicit assumptions

195/65R15 has a nominal diameter of 634.5 mm; 205/55R16 has 631.9 mm, a decrease of 2.6 mm or about 0.41%. This is not a fitment verdict.

225/45R17 and 235/45R17 have nominal diameters of 634.3 and 643.3 mm. The alternative is 9.0 mm larger, about 1.42%, with a 4.5 mm radius difference. This does not measure ground clearance.

For the first pair, an indicated 100 km/h corresponds to about 99.6 km/h in the idealized example, not a measurement of actual road speed.

Nominal comparison is not vehicle approval

Manufacturing, measuring rim, inflation, load, wear and operating conditions affect actual dimensions and rolling behavior. Geometric circumference is not a measured rolling circumference.

The tool does not check load index, speed rating, rim width, offset, clearances, brakes, suspension, drivetrain requirements or local approval rules. There is no universal safe percentage band. Follow vehicle specifications and seek qualified advice.

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The calculation does not add uploads, saved history or input-bearing links. Copy uses the clipboard only when requested. Normal site services and consent settings remain separate.