The index1,945 designations
Every tyre size,
Every tyre size,
indexed
1,945 metric designations, of which 1,220 sit inside the conventional fitment envelope. Start from the rim you have, or the width you have.
By rim diameter
The number you already know, because it is stamped on the wheel.
13-inch rim153 sizes · 88 conventional14-inch rim178 sizes · 101 conventional15-inch rim202 sizes · 109 conventional16-inch rim224 sizes · 125 conventional17-inch rim228 sizes · 167 conventional18-inch rim218 sizes · 137 conventional19-inch rim206 sizes · 137 conventional20-inch rim193 sizes · 125 conventional21-inch rim180 sizes · 121 conventional22-inch rim163 sizes · 110 conventional
By section width
The first of the three numbers, in millimetres.
135 mm66 sizes145 mm69 sizes155 mm79 sizes165 mm80 sizes175 mm89 sizes185 mm91 sizes195 mm92 sizes205 mm97 sizes215 mm99 sizes225 mm98 sizes235 mm95 sizes245 mm100 sizes255 mm99 sizes265 mm99 sizes275 mm89 sizes285 mm89 sizes295 mm83 sizes305 mm82 sizes315 mm75 sizes325 mm74 sizes335 mm68 sizes345 mm66 sizes355 mm66 sizes
How the figures are derived
- Read the size as the specification it is: section width in millimetres, aspect ratio as a percentage of that width, rim diameter in inches.
- Sidewall height = width × aspect ÷ 100. This is the number the size string is really about, and it is the one nobody quotes.
- Overall diameter = rim × 25.4 + two sidewalls — one at the top of the wheel and one at the bottom.
- Circumference = π × overall diameter, which gives revolutions per kilometre and per mile directly.
- Nothing above is measured, estimated, averaged or dated. It is the definition of the designation, so it does not go stale and it has no source to cite beyond the standard itself.
sidewall (mm) = width × aspect ÷ 100 overall diameter (mm) = rim × 25.4 + 2 × sidewall speedo true speed = indicated × (new diameter ÷ old diameter)
The full method, the ±3% rule, and what geometry cannot tell you →
Reference
Size calculatorcompare any two, with a verdict
Sidewall codesevery field decoded
Load index tablecode to kilograms
Speed symbolsletter to km/h
Plus sizingbigger wheel, same diameter
Speedometer errorthe rule, and the maths
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·1,945 designations generated from the standard's own increments; 1,220 inside the conventional envelope·REVIEWED ETRTO STANDARD