275/40R20 tyre size
728.0 mm diameter
28.66 in · 110.0 mm sidewall · 2.29 m rolling circumference · 437 rev/km
Reading 275/40R20
| 275 | Tread width in millimetres |
| 40 | Aspect ratio — sidewall height as a percentage of the width, so 275 × 40% = 110.0 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 275/40R20 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 275 mm · 10.83 in |
| Sidewall height | 110.0 mm · 4.33 in |
| Rim diameter | 20 in · 508.0 mm |
| Overall diameter | 728.0 mm · 28.66 in |
| Rolling circumference | 2287.1 mm · 2.29 m |
| Revolutions per kilometre | 437 |
| Revolutions per mile | 704 |
Sizes that fit in place of 275/40R20
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 |
|---|---|---|---|
| 315/35R20 | 728.5 mm | +0.07% | 100.1 km/h actual |
| 225/65R17 | 724.3 mm | -0.51% | 99.5 km/h actual |
| 235/65R17 | 737.3 mm | +1.28% | 101.3 km/h actual |
| 255/55R18 | 737.7 mm | +1.33% | 101.3 km/h actual |
| 235/60R18 | 739.2 mm | +1.54% | 101.5 km/h actual |
| 295/35R21 | 739.9 mm | +1.63% | 101.6 km/h actual |
| 285/35R20 | 707.5 mm | -2.82% | 97.2 km/h actual |
| 245/70R16 | 749.4 mm | +2.94% | 102.9 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 |
| 275/45R20 | 123.8 mm | 755.5 mm |
| 275/55R20 | 151.3 mm | 810.5 mm |
| 285/35R20 | 99.8 mm | 707.5 mm |
| 305/30R20 | 91.5 mm | 691.0 mm |
| 315/35R20 | 110.3 mm | 728.5 mm |
Frequently asked questions
What does 275/40R20 mean?
275 is the tread width in millimetres, 40 is the sidewall height as a percentage of that width — so 275 × 40% = 110.0 mm of sidewall — and R20 is a radial tyre on a 20-inch rim. Overall diameter is 20 × 25.4 + 2 × 110.0 = 728.0 mm.
What is the overall diameter of a 275/40R20?
728.0 mm (28.66 inches), with a rolling circumference of 2.29 m — about 437 revolutions per kilometre.
What size is equivalent to a 275/40R20?
315/35R20 (+0.07%), 225/65R17 (-0.51%) — 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.