285/35R20 tyre size
707.5 mm diameter
27.85 in · 99.8 mm sidewall · 2.22 m rolling circumference · 450 rev/km
Reading 285/35R20
| 285 | Tread width in millimetres |
| 35 | Aspect ratio — sidewall height as a percentage of the width, so 285 × 35% = 99.8 mm |
| R | Radial construction, which is almost every road tyre made since the 1970s |
| 20 | Rim diameter in inches — 508.0 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 285/35R20 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 285 mm · 11.22 in |
| Sidewall height | 99.8 mm · 3.93 in |
| Rim diameter | 20 in · 508.0 mm |
| Overall diameter | 707.5 mm · 27.85 in |
| Rolling circumference | 2222.7 mm · 2.22 m |
| Revolutions per kilometre | 450 |
| Revolutions per mile | 724 |
Sizes that fit in place of 285/35R20
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 |
|---|---|---|---|
| 245/40R20 | 704.0 mm | -0.49% | 99.5 km/h actual |
| 225/60R17 | 701.8 mm | -0.81% | 99.2 km/h actual |
| 305/30R20 | 691.0 mm | -2.33% | 97.7 km/h actual |
| 225/65R17 | 724.3 mm | +2.37% | 102.4 km/h actual |
| 235/55R17 | 690.3 mm | -2.43% | 97.6 km/h actual |
| 215/60R17 | 689.8 mm | -2.50% | 97.5 km/h actual |
| 275/40R20 | 728.0 mm | +2.90% | 102.9 km/h actual |
| 255/45R18 | 686.7 mm | -2.94% | 97.1 km/h actual |
| 255/40R19 | 686.6 mm | -2.95% | 97.0 km/h actual |
| 315/35R20 | 728.5 mm | +2.97% | 103.0 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 20-inch sizes
| Size | Sidewall | Diameter |
|---|---|---|
| 245/40R20 | 98.0 mm | 704.0 mm |
| 275/40R20 | 110.0 mm | 728.0 mm |
| 275/45R20 | 123.8 mm | 755.5 mm |
| 275/55R20 | 151.3 mm | 810.5 mm |
| 305/30R20 | 91.5 mm | 691.0 mm |
| 315/35R20 | 110.3 mm | 728.5 mm |
Frequently asked questions
What does 285/35R20 mean?
285 is the tread width in millimetres, 35 is the sidewall height as a percentage of that width — so 285 × 35% = 99.8 mm of sidewall — and R20 is a radial tyre on a 20-inch rim. Overall diameter is 20 × 25.4 + 2 × 99.8 = 707.5 mm.
What is the overall diameter of a 285/35R20?
707.5 mm (27.85 inches), with a rolling circumference of 2.22 m — about 450 revolutions per kilometre.
What size is equivalent to a 285/35R20?
245/40R20 (-0.49%), 225/60R17 (-0.81%) — 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.