Tire size calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Diameter = rim × 25.4 + 2 × width × aspect ratio ÷ 100 (mm).
- The ratio of new to old diameter scales your true speed against the speedometer reading. Changes beyond about 3% can affect ABS, gearing and legal limits.
| Inputs | Answer |
|---|---|
| Current tire size: 205/55R16; New tire size: 225/50R17; Speed shown on your speedometer: 60 | The new tire is 656.8 mm tall against 631.9 mm: 3.94% different, so your true speed is about 62.4 when the speedometer shows 60. |
| Current tire size: 225/65R17; New tire size: 245/65R17; Speed shown on your speedometer: 70 | The new tire is 750.3 mm tall against 724.3 mm: 3.59% different, so your true speed is about 72.5 when the speedometer shows 70. |
| Current tire size: 195/65R15; New tire size: 205/55R16; Speed shown on your speedometer: 50 | The new tire is 631.9 mm tall against 634.5 mm: -0.41% different, so your true speed is about 49.8 when the speedometer shows 50. |
How to use it
Overall tire diameter = rim diameter × 25.4 + 2 × width × aspect ratio ÷ 100, in millimetres. A 205/55R16 tire is about 631.9 mm tall and a 225/50R17 is about 656.8 mm, 3.94% taller, so the speedometer reads about 3.9% low and your true speed is higher than shown.
Differences beyond about 3% can affect ABS, traction control, gearing and legal speed readings; check your vehicle's manual.
Frequently asked questions
How do I read a tire size?
205/55R16: width 205 mm, sidewall 55% of the width, radial, rim 16 inches.
What happens if my tire is taller?
The speedometer reads lower than your true speed and the gearing is effectively higher.
How much difference is acceptable?
Many manufacturers suggest staying within about 3%.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.