Reading a tire size code
A metric tire size like 205/55R16 packs three measurements and a construction type into one string, and each part means something specific.
| Segment | Meaning |
|---|---|
| 205 | Section width in millimeters (tread edge to tread edge) |
| 55 | Aspect ratio: sidewall height as a % of section width |
| R | Radial construction (the near-universal standard today) |
| 16 | Wheel (rim) diameter in inches |
From the code to an actual diameter
Sidewall height mixes millimeters and a percentage, and the rim diameter is in inches — converting everything to inches and adding two sidewalls (top and bottom) to the rim gives the tire's true overall diameter.
Sidewall (in) = Width (mm) × Aspect % ÷ 100 ÷ 25.4
205/55R16: 205 × 0.55 ÷ 25.4 = 4.44 in
Overall diameter = Rim (in) + 2 × Sidewall = 16 + 8.89 = 24.89 in
From there, circumference is π × diameter, and revolutions per mile is 63,360 (inches in a mile) divided by that circumference — the number that actually determines speedometer and odometer accuracy.
Why plus-sizing keeps a lower aspect ratio on a bigger wheel
"Plus-sizing" means moving to a larger wheel diameter while deliberately choosing a lower aspect ratio tire, so the overall rolling diameter stays close to the original. A 205/55R16 (24.89 in diameter) and a 225/50R17 (roughly 25.86 in) aren't identical, but they're much closer than jumping to a 17-inch wheel with the same 55 aspect ratio would be — that mismatch is exactly what a wider, shorter-sidewall tire on the bigger wheel is designed to correct for.
How a diameter change shows up on the speedometer
A speedometer doesn't measure road speed directly — it measures wheel rotation rate and assumes a specific tire diameter to convert that into a speed reading. Change the diameter without recalibrating, and the assumption becomes wrong by the same percentage the diameter changed.
Actual speed ≈ Indicated speed × (New diameter ÷ Original diameter)
24.89 in → 25.86 in (≈4% larger), indicated 60 mph → actual ≈ 62.3 mph
That's why the common guidance is to stay within about 3% of the original overall diameter when changing tire size — beyond that, the speedometer and odometer both drift enough to matter, and in some vehicles anti-lock brakes and traction control (which also rely on wheel-speed sensors) can be affected too.
Related calculations
Once you know how far off a speedometer is running, the speed calculator can work out true speed, distance, or travel time using the corrected numbers instead of the dash reading.