305/30R20 tyre size
691.0 mm diameter
27.20 in · 91.5 mm sidewall · 2.17 m rolling circumference · 461 rev/km
Reading 305/30R20
| 305 | Tread width in millimetres |
| 30 | Aspect ratio — sidewall height as a percentage of the width, so 305 × 30% = 91.5 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 305/30R20 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 305 mm · 12.01 in |
| Sidewall height | 91.5 mm · 3.60 in |
| Rim diameter | 20 in · 508.0 mm |
| Overall diameter | 691.0 mm · 27.20 in |
| Rolling circumference | 2170.8 mm · 2.17 m |
| Revolutions per kilometre | 461 |
| Revolutions per mile | 741 |
Sizes that fit in place of 305/30R20
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 |
|---|---|---|---|
| 235/55R17 | 690.3 mm | -0.10% | 99.9 km/h actual |
| 215/60R17 | 689.8 mm | -0.17% | 99.8 km/h actual |
| 255/45R18 | 686.7 mm | -0.62% | 99.4 km/h actual |
| 255/40R19 | 686.6 mm | -0.64% | 99.4 km/h actual |
| 215/65R16 | 685.9 mm | -0.74% | 99.3 km/h actual |
| 225/60R17 | 701.8 mm | +1.56% | 101.6 km/h actual |
| 225/55R17 | 679.3 mm | -1.69% | 98.3 km/h actual |
| 245/40R20 | 704.0 mm | +1.88% | 101.9 km/h actual |
| 245/45R18 | 677.7 mm | -1.92% | 98.1 km/h actual |
| 275/35R19 | 675.1 mm | -2.30% | 97.7 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 |
| 285/35R20 | 99.8 mm | 707.5 mm |
| 315/35R20 | 110.3 mm | 728.5 mm |
Frequently asked questions
What does 305/30R20 mean?
305 is the tread width in millimetres, 30 is the sidewall height as a percentage of that width — so 305 × 30% = 91.5 mm of sidewall — and R20 is a radial tyre on a 20-inch rim. Overall diameter is 20 × 25.4 + 2 × 91.5 = 691.0 mm.
What is the overall diameter of a 305/30R20?
691.0 mm (27.20 inches), with a rolling circumference of 2.17 m — about 461 revolutions per kilometre.
What size is equivalent to a 305/30R20?
235/55R17 (-0.10%), 215/60R17 (-0.17%), 255/45R18 (-0.62%) — 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.