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Tire Size Calculator

Enter a tire size code to instantly calculate sidewall height, overall diameter, circumference and revolutions per kilometre. Compare an alternative size to see the diameter difference, speedometer deviation and load / speed index suitability.

Free · no sign-up required

Passenger, SUV, light commercial (C), heavy vehicle (22.5) and flotation sizes (31x10.50R15) are supported.

Examples

What the size code means

  • 205205 mm: section width
  • /5555%: aspect ratio (sidewall height / width)
  • RR: radial construction
  • 1616 in: rim diameter (406.4 mm)
  • 9191: load index, up to 615 kg per tire
  • VV: speed rating, up to 240 km/h
Ø 632113
Rim 16 in · Width 205 mm · Diameter 632 mm

Calculated dimensions

Sidewall height
112.8 mm
Overall diameter
631.9 mm
24.9 in
Circumference (one revolution)
1.99 m
Revolutions per km
504 rev

Compare with an alternative size

How to read a tire size

The size code on the sidewall describes the tire's dimensions and load capacity in a standard format. Using 205/55 R16 91V as an example:

205/55 R16 91V

  • 205: section width in millimetres.
  • 55: aspect ratio; the sidewall height is 55 percent of the width.
  • R: radial construction. ZR is the legacy marking for speeds above 240 km/h; D and B indicate bias construction.
  • 16: rim diameter in inches (16 × 25.4 = 406.4 mm).
  • 91: load index; up to 615 kg per tire.
  • V: speed rating; up to 240 km/h.

How is tire size calculated?

  1. Sidewall height = width × aspect ratio / 100 → 205 × 55 / 100 = 112.75 mm
  2. Rim diameter = rim (in) × 25.4 → 16 × 25.4 = 406.4 mm
  3. Overall diameter = rim diameter + 2 × sidewall height → 406.4 + 225.5 = 631.9 mm
  4. Circumference = π × overall diameter → approx. 1,985 mm
  5. Revolutions per km = 1,000,000 / circumference → approx. 504

What to check when changing tire size

  • The overall diameter difference should generally stay within ±3%.
  • The load index and speed rating must not be lower than those in the vehicle registration or the manufacturer's approval.
  • Light commercial vehicles (vans, pickups) require C-marked tires.
  • The rim width must be within the approved rim range of the new tire.
  • The vehicle manufacturer's approved size list prevails; check it before changing size.

Frequently asked questions

How is tire size calculated?

Sidewall height is width × aspect ratio / 100 and the rim diameter is inches × 25.4. The overall diameter is the rim diameter plus two sidewall heights. For 205/55 R16 the overall diameter is 631.9 mm.

Can I fit a different tire size?

Yes, if the overall diameter difference stays within ±3%, the load index and speed rating are not lower than the original and the size is on the vehicle manufacturer's approved list. If the size is recorded in the registration, check this as well.

How much does the speedometer deviate when the tire size changes?

The deviation equals the diameter difference. If the diameter grows by 2 percent, the actual speed is about 102 km/h when the speedometer shows 100 km/h. The comparison in the calculator shows this value directly.

Do 205/55 R16 and 225/45 R17 have the same diameter?

Very nearly: 205/55 R16 is 631.9 mm and 225/45 R17 is 634.3 mm. The difference is about 0.4 percent, within the ±3% limit, which is why 225/45 R17 is a common larger-rim alternative to 205/55 R16.

What do XL, C, M+S and 3PMSF mean on a tire?

XL marks a reinforced (extra load) tire, C a light commercial vehicle tire, M+S the mud and snow marking, and 3PMSF (three-peak mountain snowflake) a tire that has passed the winter performance test.

Where are the load index and speed rating?

They are the number and letter right after the size code; in 91V, 91 is the load index (615 kg) and V the speed rating (240 km/h). All values are in the Load & Speed Index table.

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