235/45R17 tyre size
643.3 mm diameter
25.33 in · 105.8 mm sidewall · 2.02 m rolling circumference · 495 rev/km
Reading 235/45R17
| 235 | Tread width in millimetres |
| 45 | Aspect ratio — sidewall height as a percentage of the width, so 235 × 45% = 105.8 mm |
| R | Radial construction, which is almost every road tyre made since the 1970s |
| 17 | Rim diameter in inches — 431.8 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 235/45R17 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 235 mm · 9.25 in |
| Sidewall height | 105.8 mm · 4.16 in |
| Rim diameter | 17 in · 431.8 mm |
| Overall diameter | 643.3 mm · 25.33 in |
| Rolling circumference | 2021.0 mm · 2.02 m |
| Revolutions per kilometre | 495 |
| Revolutions per mile | 796 |
Sizes that fit in place of 235/45R17
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 |
|---|---|---|---|
| 215/55R16 | 642.9 mm | -0.06% | 99.9 km/h actual |
| 235/40R18 | 645.2 mm | +0.30% | 100.3 km/h actual |
| 225/40R18 | 637.2 mm | -0.95% | 99.1 km/h actual |
| 205/50R17 | 636.8 mm | -1.01% | 99.0 km/h actual |
| 195/65R15 | 634.5 mm | -1.37% | 98.6 km/h actual |
| 225/45R17 | 634.3 mm | -1.40% | 98.6 km/h actual |
| 205/60R16 | 652.4 mm | +1.41% | 101.4 km/h actual |
| 245/40R18 | 653.2 mm | +1.54% | 101.5 km/h actual |
| 245/35R19 | 654.1 mm | +1.68% | 101.7 km/h actual |
| 205/55R16 | 631.9 mm | -1.77% | 98.2 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 17-inch sizes
| Size | Sidewall | Diameter |
|---|---|---|
| 205/50R17 | 102.5 mm | 636.8 mm |
| 215/45R17 | 96.8 mm | 625.3 mm |
| 215/55R17 | 118.3 mm | 668.3 mm |
| 215/60R17 | 129.0 mm | 689.8 mm |
| 225/45R17 | 101.3 mm | 634.3 mm |
| 225/50R17 | 112.5 mm | 656.8 mm |
| 225/55R17 | 123.8 mm | 679.3 mm |
| 225/60R17 | 135.0 mm | 701.8 mm |
Frequently asked questions
What does 235/45R17 mean?
235 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width — so 235 × 45% = 105.8 mm of sidewall — and R17 is a radial tyre on a 17-inch rim. Overall diameter is 17 × 25.4 + 2 × 105.8 = 643.3 mm.
What is the overall diameter of a 235/45R17?
643.3 mm (25.33 inches), with a rolling circumference of 2.02 m — about 495 revolutions per kilometre.
What size is equivalent to a 235/45R17?
215/55R16 (-0.06%), 235/40R18 (+0.30%), 225/40R18 (-0.95%) — 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.