Tire Size Calculator
Compare two tire sizes and see how they affect tire diameter and your speedometer reading.
How to use the tire size calculator
- Enter the width, aspect ratio and rim diameter for the tire set you have today.
- Enter the same measurements for the tire set you're considering switching to.
- 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:
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.
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.