🛞 Automotive
Tire & Wheel Size Calculator
Decode a tire size into diameter, sidewall, circumference and speedometer change, and compare an alternative size. Press Calculate.
About the Tire & Wheel Size Calculator
A tire size like 205/55R16 means a 205 mm section width, a sidewall height that is 55% of that width, and a 16-inch wheel diameter. The overall diameter is the wheel diameter plus two sidewalls: 16 × 25.4 + 2 × (205 × 0.55) = 631.9 mm.
Overall diameter drives the speedometer, odometer, gearing and clearance. A plus-one wheel with a shorter sidewall can keep the same overall diameter if the width and aspect ratio are chosen carefully. This calculator compares two sizes and reports the speedometer error, which is the percentage difference in distance per revolution.
How the calculation works
- Sidewall (mm) = section width × aspect ratio ÷ 100.
- Overall diameter (mm) = wheel diameter × 25.4 + 2 × sidewall.
- Circumference = π × diameter.
- Revolutions per mile = 1,609,344 ÷ circumference (mm).
- Speedometer error = (new diameter ÷ old diameter − 1) × 100.
Reference values
| Size | Diameter | Circumference | Rev/mile |
|---|---|---|---|
| 185/65R15 | 621.5 mm | 1,952 mm | 824 |
| 195/65R15 | 634.5 mm | 1,993 mm | 807 |
| 205/55R16 | 631.9 mm | 1,985 mm | 811 |
| 215/55R17 | 668.3 mm | 2,100 mm | 766 |
| 225/45R18 | 659.7 mm | 2,072 mm | 777 |
| 265/35R19 | 668.1 mm | 2,099 mm | 767 |
Worked example: 205/55R16 to 225/45R17
A 205/55R16 has a sidewall of 112.75 mm and an overall diameter of 631.9 mm. A 225/45R17 has a sidewall of 101.25 mm and a diameter of 634.3 mm—2.4 mm larger, or 0.38%. At an indicated 60 mph, the speedometer would read about 59.8 mph. Both sizes fit a rim roughly 7.0–8.5 inches wide.
Keeping the overall diameter close preserves the speedometer, gearing and clearance. A difference of a few tenths of a percent is negligible; a difference of several percent affects the odometer and can trigger clearance or gearing problems. Check the load index and speed rating as well as the size.