Quick answer: A 205/55 R16 tyre is 2 × 205 × 0.55 + 16 × 25.4 = 631.9 mm tall and a 225/45 R17 is 634.3 mm, 0.38% larger, so at 100 km/h on the speedometer you are really doing 100.4 km/h.
How it's calculated
In a metric size such as 205/55 R16, 205 is the section width in mm, 55 is the aspect ratio (sidewall height as a percentage of width) and 16 is the rim diameter in inches. Sidewall = 205 × 55% = 112.75 mm.
Overall diameter = 2 × sidewall + rim × 25.4 mm. Circumference = π × diameter, and revolutions per kilometre = 1,000,000 ÷ circumference in mm.
The speedometer is calibrated for the original size. With a different diameter, true speed = indicated speed × new diameter ÷ old diameter. UN Regulation 39 requires speedometers never to show less than the true speed, so a larger tyre can push a car outside that rule; fitters commonly keep the diameter change within about 3%.
Worked examples
- 205/55 R16 → 225/45 R17: 631.9 → 634.3 mm, +0.38%, 100 → 100.4 km/h
- 265/65 R17 → 265/70 R17: 776.3 → 802.8 mm, +3.41%
True speed by diameter change
| Diameter change | Shown 60 | Shown 100 | Shown 120 |
|---|---|---|---|
| -3% | 58.2 | 97 | 116.4 |
| -2% | 58.8 | 98 | 117.6 |
| -1% | 59.4 | 99 | 118.8 |
| +1% | 60.6 | 101 | 121.2 |
| +2% | 61.2 | 102 | 122.4 |
| +3% | 61.8 | 103 | 123.6 |
Frequently asked questions
What do the numbers on a tyre mean?
In 205/55 R16, 205 mm is the width, 55 is the sidewall height as a percentage of the width, R means radial construction and 16 is the rim diameter in inches.
How much can tyre diameter change?
A change within about 3% is the usual fitting guideline, and smaller is better. Check the sizes approved for your car in the owner's manual or the door-pillar placard.
Does a bigger tyre change fuel use?
It can: wider and heavier tyres add rolling resistance and weight, and the odometer reads short, so fuel economy worked out from it looks better than it is.
What do the load index and speed rating mean?
Take 91V. The 91 is the load index, 615 kg per tyre, and V is the speed rating, good for 240 km/h. A replacement should match or beat what your car's manual lists.
How do I find out how old a tyre is?
Look for the DOT code on the sidewall. The last four digits are the week and year it was made, so 2324 means week 23 of 2024. Rubber ages faster in Gulf heat, so look older tyres over carefully and stick to the maker's age limit.
Is 225/45 R17 the same height as 205/55 R16?
Very nearly. It's 634.3 mm against 631.9 mm, only 0.4% bigger, which is why it's a popular plus-one swap. At an indicated 100 km/h you'd really be doing about 100.4.