Tire size calculator: Compare tire sizes and rolling circumference
If you are considering a different tire size, you can start by comparing the geometry. The calculator shows the diameter, sidewall height and rolling circumference for two sizes and the percentage difference between them.
Worked example
- Width: 205 mm
- Profile: 55 %
- Rim: 16 inches
- Width: 225 mm
- Profile: 45 %
- Rim: 17 inches
Difference in diameter
0.3798 %
If the speedometer shows 100 km/h, you drive about 100.3798 km/h with the new tires (theoretically).
| Current | New | |
|---|---|---|
| Sidewall | 112.75 mm | 101.25 mm |
| Diameter | 631.9 mm | 634.3 mm |
| Perimeter | 1,985.1724 mm | 1,992.7122 mm |
The same rolling circumference does not mean the wheel fits. Load and speed rating, rim, clearance and the car's approval must also be in order.
How to read a tire size
In the size 205/55 R16, 205 is the nominal width of the tire in millimeters. The aspect ratio 55 gives the sidewall height as a percentage of the width, and 16 is the rim diameter in inches. The calculator converts the measurements to the same unit.
The diameter consists of the rim diameter and two sidewalls. The rolling circumference is calculated as the diameter times pi. It is a geometric approximation; the actual rolling circumference of a loaded tire can differ.
Guide to comparing tires
Use the current size as a reference
First enter the approved size you are comparing with. Then add the alternative. The speed display describes the theoretical change from the reference set and does not know the original deviation of the car's speedometer.
Example of diameter
For 205/55 R16, the sidewall is 112.75 mm. The rim is 406.4 mm, so the calculated diameter is 631.9 mm. If you compare the size with itself, the difference is zero.
The same rolling circumference does not automatically mean that a wheel fits. Load index, speed rating, rim, clearance and the car's approval must also be in order. Check the information for your particular car before you buy. Use the car budget if you want to spread tire costs over the time you own the car.