225/40R18 tyre size
637.2 mm diameter
25.09 in · 90.0 mm sidewall · 2.00 m rolling circumference · 500 rev/km
Reading 225/40R18
| 225 | Tread width in millimetres |
| 40 | Aspect ratio — sidewall height as a percentage of the width, so 225 × 40% = 90.0 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 225/40R18 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 225 mm · 8.86 in |
| Sidewall height | 90.0 mm · 3.54 in |
| Rim diameter | 18 in · 457.2 mm |
| Overall diameter | 637.2 mm · 25.09 in |
| Rolling circumference | 2001.8 mm · 2.00 m |
| Revolutions per kilometre | 500 |
| Revolutions per mile | 804 |
Sizes that fit in place of 225/40R18
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 |
|---|---|---|---|
| 205/50R17 | 636.8 mm | -0.06% | 99.9 km/h actual |
| 195/65R15 | 634.5 mm | -0.42% | 99.6 km/h actual |
| 225/45R17 | 634.3 mm | -0.46% | 99.5 km/h actual |
| 205/55R16 | 631.9 mm | -0.83% | 99.2 km/h actual |
| 215/55R16 | 642.9 mm | +0.89% | 100.9 km/h actual |
| 235/45R17 | 643.3 mm | +0.96% | 101.0 km/h actual |
| 235/40R18 | 645.2 mm | +1.26% | 101.3 km/h actual |
| 215/45R17 | 625.3 mm | -1.87% | 98.1 km/h actual |
| 205/60R16 | 652.4 mm | +2.39% | 102.4 km/h actual |
| 185/65R15 | 621.5 mm | -2.46% | 97.5 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 18-inch sizes
| Size | Sidewall | Diameter |
|---|---|---|
| 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 |
| 255/45R18 | 114.8 mm | 686.7 mm |
| 255/55R18 | 140.3 mm | 737.7 mm |
| 265/60R18 | 159.0 mm | 775.2 mm |
Frequently asked questions
What does 225/40R18 mean?
225 is the tread width in millimetres, 40 is the sidewall height as a percentage of that width — so 225 × 40% = 90.0 mm of sidewall — and R18 is a radial tyre on a 18-inch rim. Overall diameter is 18 × 25.4 + 2 × 90.0 = 637.2 mm.
What is the overall diameter of a 225/40R18?
637.2 mm (25.09 inches), with a rolling circumference of 2.00 m — about 500 revolutions per kilometre.
What size is equivalent to a 225/40R18?
205/50R17 (-0.06%), 195/65R15 (-0.42%), 225/45R17 (-0.46%) — 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.