Tire size calculator
Decode a marking such as 205/55R16 91V into sidewall height, overall diameter, circumference and revolutions per kilometer, then compare it with a reference size for speedometer error and clearance change.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Tire markings
Overall diameter = rim diameter × 25.4 + 2 × section width × aspect ratio; the circumference is π × diameter and the revolutions follow from it. Load index and speed rating tables follow ETRTO.
Result
664.40 mm · 2087.27 mm
129.00 mm
479.1 /km
631.90 mm · 1985.17 mm
503.7 /km
95 → 690 kg per tire
H → up to 210 km/h
32.50 mm (5.14%)
5.14%
105.14 km/h
16.25 mm
-24.6 /km
What this tool does
- Check the size difference before buying tires: see how far a larger or smaller tire throws off the speedometer and odometer.
- Decode a marking like 205/55R16 91V: section width, aspect ratio, rim diameter, load index 91 (615 kg) and speed rating V (240 km/h).
- Plan a wheel upgrade: 205/55R16 to 225/45R17 differs by only 2.4 mm in diameter, so the speedometer barely moves.
- Get the revolutions per kilometer — about 503.7 for 205/55R16 — for navigation or odometer calibration.
Example
Input
Original 205/55R16, new tire 215/60R16 95H, speedometer reading 100 km/h
Output
New overall diameter 664.40 mm (circumference 2087.27 mm, 479.1 /km), original 631.90 mm (circumference 1985.17 mm, 503.7 /km), diameter difference +32.50 mm (+5.14%), speedometer error +5.14%, actual speed about 105.14 km/h at an indicated 100 km/h, ride height +16.25 mm
The diameter of 205/55R16 is 16 × 25.4 + 2 × 205 × 55% = 406.4 + 225.5 = 631.9 mm, matching the industry figure for that size.
Frequently asked questions
Why does the speedometer go wrong after a tire change?
The speedometer is calibrated to the circumference of the original tire. A larger diameter means a higher true speed for the same wheel rpm, so an indicated 100 km/h becomes about 105.14 km/h (+5.14%) — and speeding tickets get easier. A smaller tire makes the reading optimistic instead.
How much diameter difference is safe?
The usual rule of thumb is to stay within ±3%. Beyond that you affect the speedometer, odometer, gearbox shift logic and the wheel-speed signals used by ABS and ESP; past 5% you can also risk arch contact or upset the suspension geometry.
What do load index 91 and speed rating V mean?
A load index of 91 means each tire can carry 615 kg, so four tires cover 2460 kg; the V rating allows a sustained maximum of 240 km/h. Neither figure should be lower than the original specification — match or exceed it.
How are circumference and revolutions per kilometer derived?
Circumference = π × overall diameter, and revolutions per kilometer = 1 000 000 mm ÷ circumference. For 205/55R16 that is about 1985.17 mm and roughly 503.7 revolutions per kilometer. Under load the tire deflects, so the effective circumference is slightly smaller — treat the figure as theoretical.
Can the rim diameter be a decimal?
Yes. The tool accepts values such as 15.5 (used by some light truck and off-road tires). Section width is limited to 100–400 mm, aspect ratio to 20–100% and rim diameter to 8–26 inches; anything outside that range reports an error.
Keywords：tire size calculatortyre size205/55R16speedometer errortire diameterload index轮胎尺寸计算轮胎规格速度表偏差载重指数轮胎直径