255/45R18 tyre size
686.7 mm diameter
27.04 in · 114.8 mm sidewall · 2.16 m rolling circumference · 464 rev/km
Reading 255/45R18
| 255 | Tread width in millimetres |
| 45 | Aspect ratio — sidewall height as a percentage of the width, so 255 × 45% = 114.8 mm |
| R | Radial construction, which is almost every road tyre made since the 1970s |
| 18 | Rim diameter in inches — 457.2 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 255/45R18 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 255 mm · 10.04 in |
| Sidewall height | 114.8 mm · 4.52 in |
| Rim diameter | 18 in · 457.2 mm |
| Overall diameter | 686.7 mm · 27.04 in |
| Rolling circumference | 2157.3 mm · 2.16 m |
| Revolutions per kilometre | 464 |
| Revolutions per mile | 746 |
Sizes that fit in place of 255/45R18
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 |
|---|---|---|---|
| 255/40R19 | 686.6 mm | -0.01% | 100.0 km/h actual |
| 215/65R16 | 685.9 mm | -0.12% | 99.9 km/h actual |
| 215/60R17 | 689.8 mm | +0.45% | 100.5 km/h actual |
| 235/55R17 | 690.3 mm | +0.52% | 100.5 km/h actual |
| 305/30R20 | 691.0 mm | +0.63% | 100.6 km/h actual |
| 225/55R17 | 679.3 mm | -1.08% | 98.9 km/h actual |
| 245/45R18 | 677.7 mm | -1.31% | 98.7 km/h actual |
| 275/35R19 | 675.1 mm | -1.69% | 98.3 km/h actual |
| 205/65R16 | 672.9 mm | -2.01% | 98.0 km/h actual |
| 225/60R17 | 701.8 mm | +2.20% | 102.2 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 18-inch sizes
| Size | Sidewall | Diameter |
|---|---|---|
| 225/40R18 | 90.0 mm | 637.2 mm |
| 235/40R18 | 94.0 mm | 645.2 mm |
| 235/60R18 | 141.0 mm | 739.2 mm |
| 245/40R18 | 98.0 mm | 653.2 mm |
| 245/45R18 | 110.3 mm | 677.7 mm |
| 255/55R18 | 140.3 mm | 737.7 mm |
| 265/60R18 | 159.0 mm | 775.2 mm |
Frequently asked questions
What does 255/45R18 mean?
255 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width — so 255 × 45% = 114.8 mm of sidewall — and R18 is a radial tyre on a 18-inch rim. Overall diameter is 18 × 25.4 + 2 × 114.8 = 686.7 mm.
What is the overall diameter of a 255/45R18?
686.7 mm (27.04 inches), with a rolling circumference of 2.16 m — about 464 revolutions per kilometre.
What size is equivalent to a 255/45R18?
255/40R19 (-0.01%), 215/65R16 (-0.12%), 215/60R17 (+0.45%) — 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.