185/65R15 tyre size
621.5 mm diameter
24.47 in · 120.3 mm sidewall · 1.95 m rolling circumference · 512 rev/km
Reading 185/65R15
| 185 | Tread width in millimetres |
| 65 | Aspect ratio — sidewall height as a percentage of the width, so 185 × 65% = 120.3 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 185/65R15 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 | 120.3 mm · 4.73 in |
| Rim diameter | 15 in · 381.0 mm |
| Overall diameter | 621.5 mm · 24.47 in |
| Rolling circumference | 1952.5 mm · 1.95 m |
| Revolutions per kilometre | 512 |
| Revolutions per mile | 824 |
Sizes that fit in place of 185/65R15
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 |
|---|---|---|---|
| 195/55R16 | 620.9 mm | -0.10% | 99.9 km/h actual |
| 215/45R17 | 625.3 mm | +0.61% | 100.6 km/h actual |
| 195/60R15 | 615.0 mm | -1.05% | 99.0 km/h actual |
| 205/55R16 | 631.9 mm | +1.67% | 101.7 km/h actual |
| 225/45R17 | 634.3 mm | +2.06% | 102.1 km/h actual |
| 195/65R15 | 634.5 mm | +2.09% | 102.1 km/h actual |
| 205/50R17 | 636.8 mm | +2.46% | 102.5 km/h actual |
| 225/40R18 | 637.2 mm | +2.53% | 102.5 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 |
|---|---|---|
| 185/55R15 | 101.8 mm | 584.5 mm |
| 195/60R15 | 117.0 mm | 615.0 mm |
| 195/65R15 | 126.8 mm | 634.5 mm |
Frequently asked questions
What does 185/65R15 mean?
185 is the tread width in millimetres, 65 is the sidewall height as a percentage of that width — so 185 × 65% = 120.3 mm of sidewall — and R15 is a radial tyre on a 15-inch rim. Overall diameter is 15 × 25.4 + 2 × 120.3 = 621.5 mm.
What is the overall diameter of a 185/65R15?
621.5 mm (24.47 inches), with a rolling circumference of 1.95 m — about 512 revolutions per kilometre.
What size is equivalent to a 185/65R15?
195/55R16 (-0.10%), 215/45R17 (+0.61%) — 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.