185/55R15 tyre size
584.5 mm diameter
23.01 in · 101.8 mm sidewall · 1.84 m rolling circumference · 545 rev/km
Reading 185/55R15
| 185 | Tread width in millimetres |
| 55 | Aspect ratio — sidewall height as a percentage of the width, so 185 × 55% = 101.8 mm |
| R | Radial construction, which is almost every road tyre made since the 1970s |
| 15 | Rim diameter in inches — 381.0 mm |
The aspect ratio is the part that catches people: it is a percentage of the width, not a fixed height. Widen the tread and keep the same aspect number and the tyre also gets taller. That is why 185/55R15 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 185 mm · 7.28 in |
| Sidewall height | 101.8 mm · 4.01 in |
| Rim diameter | 15 in · 381.0 mm |
| Overall diameter | 584.5 mm · 23.01 in |
| Rolling circumference | 1836.3 mm · 1.84 m |
| Revolutions per kilometre | 545 |
| Revolutions per mile | 876 |
Sizes that fit in place of 185/55R15
A replacement is judged on rolling diameter, because that is what the speedometer, the ABS and the gearing all depend on. Anything within about 1% is treated as interchangeable; beyond 3% the speedometer error stops being ignorable.
| Size | Diameter | Difference | Speedometer at an indicated 100 |
|---|---|---|---|
| 175/65R14 | 583.1 mm | -0.24% | 99.8 km/h actual |
| 185/60R14 | 577.6 mm | -1.18% | 98.8 km/h actual |
| 165/65R14 | 570.1 mm | -2.46% | 97.5 km/h actual |
| 175/70R14 | 600.6 mm | +2.75% | 102.8 km/h actual |
Diameter is only half the question — the replacement also has to suit the rim width and the vehicle's load and speed ratings. Check the placard in the door frame before buying.
Other 15-inch sizes
| Size | Sidewall | Diameter |
|---|---|---|
| 185/65R15 | 120.3 mm | 621.5 mm |
| 195/60R15 | 117.0 mm | 615.0 mm |
| 195/65R15 | 126.8 mm | 634.5 mm |
Frequently asked questions
What does 185/55R15 mean?
185 is the tread width in millimetres, 55 is the sidewall height as a percentage of that width — so 185 × 55% = 101.8 mm of sidewall — and R15 is a radial tyre on a 15-inch rim. Overall diameter is 15 × 25.4 + 2 × 101.8 = 584.5 mm.
What is the overall diameter of a 185/55R15?
584.5 mm (23.01 inches), with a rolling circumference of 1.84 m — about 545 revolutions per kilometre.
What size is equivalent to a 185/55R15?
175/65R14 (-0.24%) — all within 1% of this diameter, which is the band inside which a manufacturer will list alternatives for the same car. Width and rim differ; rolling diameter does not.
How much does changing tyre size affect the speedometer?
A speedometer counts wheel revolutions, so it reads in proportion to the rolling diameter it was calibrated for. Fitting a tyre 1% larger makes the speedometer read 1% low — at an indicated 100 km/h you are doing 101. Most jurisdictions expect a speedometer to never under-read, which is why replacements are kept within about 3% and why the factory tyre usually reads a little high to begin with.