Kalkulo / Car & Travel / Tire Size Calculator

Tire Size Calculator

Compare two tire sizes and see how they affect tire diameter and your speedometer reading.

Tire set 1 (current)
Tire set 2 (new/comparison)
Speed
Actual speed with tire set 2
102.4 km/h
At 100 km/h shown on the speedometer
Diameter of tire set 1
63.19 cm
Diameter of tire set 2
64.68 cm

How to use the tire size calculator

  1. Enter the width, aspect ratio and rim diameter for the tire set you have today.
  2. Enter the same measurements for the tire set you're considering switching to.
  3. Enter what your speedometer shows — the calculator works out the actual speed with the new tire set.

How tire diameter and speed are calculated

A tire's total diameter is found by adding the rim diameter and twice the sidewall height (width × aspect ratio). The actual speed is then calculated from the ratio between the two diameters:

Diameter (mm) = (Width × Aspect ratio ÷ 100 × 2) + (Rim diameter × 25.4)
Actual speed = Displayed speed × (Diameter 2 ÷ Diameter 1)

A 205/55R16 tire has a diameter of about 63.2 cm, while a 215/50R17 tire has a diameter of about 64.7 cm. If the speedometer shows 100 km/h with the new, slightly larger tire set, you're actually driving at roughly 102.4 km/h.

Note: Speedometers are typically calibrated to never read lower than the actual speed — they carry a built-in positive margin. If you switch to tires with a larger diameter than the original, the real-world deviation can become even bigger than usual, so it's worth checking against GPS speed in practice.

Frequently asked questions

What do the numbers in a tire size like 205/55R16 mean?

205 is the tire's width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 16 is the rim diameter in inches.

How much can I change the tire diameter without problems?

A common rule of thumb is to keep the diameter difference within ± 2–3% of the original diameter, to avoid errors in the speedometer, odometer and ABS/traction control systems.

Why does the speedometer usually read a little high?

Speedometers are typically designed and calibrated so that the displayed speed is never lower than the actual speed, only equal to or higher. That's why most speedometers show a bit more than the real speed.