A tyre marked 205/55 R16 is not describing its diameter anywhere. The 205 is the section width in millimetres, the 55 is the aspect ratio — sidewall height as a percentage of that width — and the 16 is the rim diameter in inches. The overall diameter has to be assembled from those three numbers, which is why fitting a different size is easy to get wrong.
Overall diameter in millimetres is the rim diameter in inches multiplied by 25.4 to convert to millimetres, plus twice the sidewall height, because there is a sidewall above and below the rim. Sidewall height is the section width multiplied by the aspect ratio over 100. For 205/55 R16 that is 16 x 25.4 + 2 x (205 x 55/100) = 631.9 mm.
Rolling circumference follows as pi times the diameter, 1985.2 mm here, which sets how many revolutions a mile takes and therefore what the speedometer reads. Fitting 225/45 R17 instead gives 634.3 mm, +0.38 per cent different, so an indicated 70 mph would truly be 70.3 mph. Manufacturers generally keep replacements within about three per cent for exactly this reason.
The formula
width- Section width in millimetres, the first number on the sidewall
aspect- Aspect ratio as a percentage of the width, the second number
rim- Wheel diameter in inches, the number after the R
25.4- Millimetres in one inch, exact
How to use this calculator
- Read the three numbers off the sidewall, for example 205/55 R16.
- Enter width in millimetres, aspect ratio as a percentage and rim size in inches.
- Enter a second size if you are considering changing wheels or tyres.
- The gauge shows overall diameter; circumference, revolutions and speedo error sit below.
Worked examples
205/55 R16 worked through
The rim is 16 x 25.4 = 406.4 mm. Each sidewall is 205 x 55/100 = 112.8 mm, and there are two of them, so the overall diameter is 631.9 mm. Circumference is pi x 631.9 = 1985.2 mm, which is 811 revolutions per mile.
Changing to 225/45 R17
The larger rim is offset by the shorter sidewall: 431.8 mm plus 2 x 101.2 mm gives 634.3 mm, only +0.38 per cent different. An indicated 70 mph becomes a true 70.3 mph, and the odometer would under-record by the same proportion.
How to read your score
Frequently asked questions
Why is the rim in inches and everything else in millimetres?
Historical accident. Rim diameters were standardised in inches long before tyre sections were described in millimetres, and the industry never reconciled the two. The conversion is exact — one inch is 25.4 mm — so the mixed units cause no loss of accuracy, only confusion.
How much size change is acceptable?
Most manufacturers work to a tolerance of about three per cent in rolling circumference. Beyond that the speedometer error becomes significant, ABS and stability systems see wheel speeds they were not calibrated for, and on some cars the tyre fouls the arch on full lock.
Which way does a bigger tyre push the speedometer?
A larger diameter turns fewer times for the same road distance, so the speedometer under-reads: you are going faster than it says. A smaller tyre does the opposite. Speedometers are also legally permitted to over-read, so most cars already show a few per cent optimistic.
Does the load index change with size?
It can, and it matters. The letters and numbers after the size, such as 91V, give load capacity and speed rating. A replacement must meet or exceed the manufacturer figure, regardless of whether the diameter matches.
Is the tread depth included in this diameter?
The calculation uses the nominal new-tyre geometry. A tyre worn from 8 mm to the 1.6 mm legal limit loses about 12.8 mm of diameter, roughly two per cent, which is enough to nudge the odometer and to matter if you are comparing a new tyre against a worn one.
Common tyre size reference
| Size | Sidewall | Diameter | Circumference | Revs per mile |
|---|---|---|---|---|
| 155/70 R13 | 108.5 mm | 547.2 mm | 1719 mm | 936 |
| 175/65 R14 | 113.8 mm | 583.1 mm | 1832 mm | 879 |
| 185/60 R15 | 111.0 mm | 603.0 mm | 1894 mm | 850 |
| 195/65 R15 | 126.8 mm | 634.5 mm | 1993 mm | 807 |
| 205/55 R16 | 112.8 mm | 631.9 mm | 1985 mm | 811 |
| 215/60 R16 | 129.0 mm | 664.4 mm | 2087 mm | 771 |
| 225/45 R17 | 101.2 mm | 634.3 mm | 1993 mm | 808 |
| 235/45 R18 | 105.8 mm | 668.7 mm | 2101 mm | 766 |
| 245/40 R19 | 98.0 mm | 678.6 mm | 2132 mm | 755 |
| 265/60 R18 | 159.0 mm | 775.2 mm | 2435 mm | 661 |
Diameter = rim x 25.4 + 2 x sidewall. Nominal new-tyre geometry; a worn tyre measures a little less.