Tire size comparison calculator
Type the current size and the new size (printed on the tire sidewall): you see the diameter difference and the effect on the speedometer.
Result
0.38 %Diameter difference
- Current diameter
- 631.9 mm
- New diameter
- 634.3 mm
- Difference in millimeters
- 2.4 mm
- True speed with the new tires
- 100.38 km/h
- Current sidewall height
- 112.75 mm
- New sidewall height
- 101.25 mm
- Revolutions per km (current)
- 503.7
- Revolutions per km (new)
- 501.8
Inputs
- Current size
- 205/55 R16
- New size
- 225/45 R17
- Speedometer reading
- 100
How it is calculated
205/55 R16: width 205 mm, sidewall = 55 % of the width, 16-inch rim.
Result
Diameter difference
- Current diameter
- 631.9 mm
- New diameter
- 634.3 mm
- Difference in millimeters
- 2.4 mm
How it is calculated
- 205/55 R16: width 205 mm, sidewall = 55 % of the width, 16-inch rim.
- Sidewall height = width × aspect ratio ÷ 100. Diameter = rim × 25.4 + 2 × sidewall.
- The speedometer is set for the current size: true speed = reading × new diameter ÷ old diameter.
Example
205/55 R16 (631.9 mm) vs 225/45 R17 (634.3 mm): +2.4 mm, so +0.38 %. When the speedometer shows 100 km/h, you drive at 100.38 km/h.
Good to know
- This is not an approval: only the sizes allowed by the car maker or the registration papers are legal.
- Load index and speed rating must also be respected.
- The real diameter varies a little with brand, wear and pressure.
Sources
- ETRTO, Standards Manual (metric tire size designation)
Frequently asked questions
What is this tool used for?
Type the current size and the new size (printed on the tire sidewall): you see the diameter difference and the effect on the speedometer.
How is the result calculated?
205/55 R16: width 205 mm, sidewall = 55 % of the width, 16-inch rim.
What should I check before using the result?
This is not an approval: only the sizes allowed by the car maker or the registration papers are legal.