Tire Size Calculator
Enter two tire sizes to get the overall diameter of each and the theoretical difference between the speedometer reading and the real speed when you change from one to the other.
How to read a tire size
205/55R16 means:
- 205: the nominal section width of the tire (mm)
- 55: the aspect ratio (the height of the sidewall as a percentage of the section width)
- R: radial construction
- 16: the diameter of the rim the tire fits (inches)
ZR or - in place of R are also read. Sizes written in inches (such as 31x10.5R15), LT (light truck) sizes, motorcycle sizes, and codes that include a load index or a speed symbol are not supported by this calculator. Enter only the size part.
The formula for the overall diameter
Overall diameter (mm) = 2 × section width × aspect ratio ÷ 100 + rim diameter (inches) × 25.4
For example, 195/55R16 gives 2 × 195 × 55 ÷ 100 + 16 × 25.4 = 620.9 mm. This is the theoretical overall diameter; the formula is the one given in a reference article (Wikipedia, "Speedometer", checked November 1, 2026), which gives the same formula (T: section width, A: aspect ratio in percent, W: rim diameter in inches, with the result in mm) and a worked example. The "Tire code" article (checked the same day, the part that could be read) explains the meaning of the size marking. The real overall diameter of a tire can differ from the theoretical value, so check the manufacturer's documentation.
The theoretical difference in speed
Suppose the speedometer is calibrated for the overall diameter of the current tire. When the diameter changes, the distance covered per turn of the wheel changes, even at the same rotation speed.
Real speed = indicated speed × new overall diameter ÷ current overall diameter
- If the new tire is larger in diameter, the real speed is higher than the speedometer shows.
- If the new tire is smaller in diameter, the real speed is lower than the speedometer shows.
- The odometer is off by the same ratio, so the real distance covered differs.
The worked example in the reference article (Wikipedia, "Speedometer"): with a standard diameter of 614.6 mm and another tire of 576.0 mm, the speedometer reads 6.7% higher than the real speed (at a real 100 km/h it shows 106.7 km/h). This calculator uses the same direction of calculation as that example (real speed = indicated speed × new diameter ÷ current diameter). The same article also prints a different expression for the "percentage error" (100 × (1 − new diameter / standard diameter)), which gives a different number for the same pair of tires; this calculator does not use that expression.
Theoretical speed difference vs. real-world speedometer error
| What it is | |
|---|---|
| Theoretical difference in speed (this calculator) | The difference between the speedometer reading and the real speed calculated from the change in tire diameter alone |
| Real-world speedometer error (not calculated) | The error in an actual vehicle, including the tolerance of the speedometer itself, differences between vehicles, tire wear, pressure, temperature, load, and the real tire size (which differs by manufacturer and rim width) |
The reference article lists tire wear, temperature, pressure, vehicle load and the nominal tire size as causes of speedometer error. The result of this calculator does not show the accuracy of a real vehicle's speedometer. Check your real speed with another method, such as GPS.
This calculator does not judge whether a tire size complies with any law or vehicle inspection standard. It also does not list any permitted tolerance. Rules differ by country and region.
Examples
| Current → new | Result |
|---|---|
| 205/55R16 (631.9 mm) → 225/45R17 (634.3 mm) | diameter +2.4 mm (about +0.38%). With 100 km/h indicated, the theoretical real speed is about 100.38 km/h |
| 195/55R16 (620.9 mm) → the same size | no difference in diameter, no difference in speed |
What is not covered
Real-world speedometer error, comparison with GPS, whether a wheel and tire combination fits (rim width), legal or inspection checks, effects on gearing, engine speed and fuel economy, and reading the tire type, load index and speed symbol are not covered. To convert speeds, use the Speed Converter; to convert lengths, use the Length Converter; for bicycle speed, use the Bicycle Gear Ratio and Speed Calculator.
Sources
- Wikipedia, "Speedometer": https://en.wikipedia.org/wiki/Speedometer (checked November 1, 2026: the formula for the overall diameter, the worked example and the causes of speedometer error)
- Wikipedia, "Tire code": https://en.wikipedia.org/wiki/Tire_code (checked November 1, 2026, for the part of the page that could be read: the meaning of the size marking)
These are explanatory references, not standards.
Frequently asked questions
What happens to the speedometer if I fit larger tires?
With a larger overall diameter, the real speed is, in theory, higher than the speedometer shows (the speedometer reads lower than the real speed). Real-world speedometer error is not calculated here.
Is the calculated diameter the same as the real tire?
It is a theoretical overall diameter. Check the real value in the manufacturer's documentation.
Will it pass an inspection?
This calculator does not judge compliance with any law or inspection standard.