Tyre Size Calculator and Comparison
Compare two tyre sizes: the overall diameter, sidewall height, the difference between them and how far your speedometer will be out.
The first number on the tyre: 195 in 195/65R15.
Difference in overall diameter
−0.41%
- True speed when the speedometer shows 100
- 99.6
- 195/65R15 diameter
- 634.5 mm (24.98 in)
- 205/55R16 diameter
- 631.9 mm (24.88 in)
- Within 3%?
- Yes, usually a safe swap
205/55R16 is 0.41% shorter than 195/65R15, so at a speedometer reading of 100 you'd really be doing 99.6. That's within the 3% most fitters accept.
How it's worked out
- 195/65R15 sidewall
195 mm × 65%126.8 mm - 195/65R15 diameter
15 in × 25.4 + 2 × 126.8634.5 mm - 205/55R16 sidewall
205 mm × 55%112.8 mm - 205/55R16 diameter
16 in × 25.4 + 2 × 112.8631.9 mm - Difference
631.9 ÷ 634.5 − 1-0.41% - True speed
100 × 631.9 ÷ 634.599.6
Both sizes side by side
| 195/65R15 | 205/55R16 | |
|---|---|---|
| Sidewall height | 126.8 mm | 112.8 mm |
| Overall diameter | 634.5 mm | 631.9 mm |
| Circumference | 1,993 mm | 1,985 mm |
| Turns per km | 502 | 504 |
- Check the new size's load index and speed rating match or beat your car's, and that it clears the arches on full lock. Your car's manual or door sticker lists approved sizes.
How it is worked out
Diameter = rim × 25.4 + 2 × width × aspect ÷ 100
The sidewall height is the width times the aspect ratio. The overall diameter is the rim plus two sidewalls. The true speed is the speedometer reading times the new diameter divided by the old.
Before you change size
Check your car's manual or the sticker in the door frame for approved sizes. Match or beat the load index and speed rating, and make sure the tyre clears the suspension and arches on full steering lock.
Questions people ask
What does 205/55R16 mean?
205 mm wide, a sidewall 55% of the width (112.8 mm) high, a radial tyre for a 16-inch rim. Its overall diameter is 631.9 mm.
Can I fit 205/55R16 instead of 195/65R15?
By diameter, yes: it's 0.41% smaller, well within the usual 3%. At an indicated 100 you'd be doing 99.6. You also need 16-inch wheels and a matching load and speed rating.
How much bigger can my new tyres be?
Most fitters accept up to about 3% difference in overall diameter. Beyond that, the speedometer, ABS and gearing are noticeably affected and the tyre may rub.
Do bigger tyres make the speedometer read low?
Yes. A taller tyre covers more ground each turn, so you're going faster than the speedometer shows; a shorter tyre makes it read high.
About this calculator
Formula and rules last checked 9 November 2026Written and tested by the CalculatorDex teamReport a problem