255/40R19 tyre size
686.6 mm diameter
27.03 in · 102.0 mm sidewall · 2.16 m rolling circumference · 464 rev/km
Reading 255/40R19
| 255 | Tread width in millimetres |
| 40 | Aspect ratio — sidewall height as a percentage of the width, so 255 × 40% = 102.0 mm |
| R | Radial construction, which is almost every road tyre made since the 1970s |
| 19 | Rim diameter in inches — 482.6 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 255/40R19 and a size with a smaller aspect number on a larger rim can be the same overall diameter.
The numbers
| Tread width | 255 mm · 10.04 in |
| Sidewall height | 102.0 mm · 4.02 in |
| Rim diameter | 19 in · 482.6 mm |
| Overall diameter | 686.6 mm · 27.03 in |
| Rolling circumference | 2157.0 mm · 2.16 m |
| Revolutions per kilometre | 464 |
| Revolutions per mile | 746 |
Sizes that fit in place of 255/40R19
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 |
|---|---|---|---|
| 255/45R18 | 686.7 mm | +0.01% | 100.0 km/h actual |
| 215/65R16 | 685.9 mm | -0.10% | 99.9 km/h actual |
| 215/60R17 | 689.8 mm | +0.47% | 100.5 km/h actual |
| 235/55R17 | 690.3 mm | +0.54% | 100.5 km/h actual |
| 305/30R20 | 691.0 mm | +0.64% | 100.6 km/h actual |
| 225/55R17 | 679.3 mm | -1.06% | 98.9 km/h actual |
| 245/45R18 | 677.7 mm | -1.30% | 98.7 km/h actual |
| 275/35R19 | 675.1 mm | -1.67% | 98.3 km/h actual |
| 205/65R16 | 672.9 mm | -2.00% | 98.0 km/h actual |
| 225/60R17 | 701.8 mm | +2.21% | 102.2 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 19-inch sizes
| Size | Sidewall | Diameter |
|---|---|---|
| 245/35R19 | 85.8 mm | 654.1 mm |
| 255/35R19 | 89.3 mm | 661.1 mm |
| 275/35R19 | 96.3 mm | 675.1 mm |
Frequently asked questions
What does 255/40R19 mean?
255 is the tread width in millimetres, 40 is the sidewall height as a percentage of that width — so 255 × 40% = 102.0 mm of sidewall — and R19 is a radial tyre on a 19-inch rim. Overall diameter is 19 × 25.4 + 2 × 102.0 = 686.6 mm.
What is the overall diameter of a 255/40R19?
686.6 mm (27.03 inches), with a rolling circumference of 2.16 m — about 464 revolutions per kilometre.
What size is equivalent to a 255/40R19?
255/45R18 (+0.01%), 215/65R16 (-0.10%), 215/60R17 (+0.47%) — 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.