255/65R18 tire size — 31.05 in, against two tires marked 31
Where the metric and imperial notations meet, this size is the one that has to be worked out.
Metric notation · Crossover and SUV
- Overall diameter
- 31.05 in
- 789 mm
- Section width
- 10.04 in
- 255 mm
- Sidewall height
- 6.53 in
- 166 mm
- Aspect ratio
- 65%
- sidewall as a share of width
- Wheel diameter
- 18 in
- the hole in the middle
- Circumference
- 97.6 in
- 2478 mm
- Revolutions per mile
- 649.5
- geometric, not measured
- Revolutions per km
- 403.6
- geometric, not measured
- Typical load index
- 98–116
- 1653–2756 lb per tire
Crossovers are heavier than the cars they share size families with, so the same nominal size is produced at a higher index for SUV fitment than for passenger fitment. Two tires wearing the same size code can carry materially different loads.
The load-index range shown is the band this CLASS of size is typically produced in — not a list of what is in stock for this exact size today. Load index and speed symbol are printed on the sidewall of the specific tire you are holding, and the required minimum is on the placard of the specific vehicle you are fitting. Read both.
Nearest real sizes in imperial notation, same wheel
- 33x12.50R1833.00 in tall+1.95 in
- 35x12.50R1835.00 in tall+3.95 in
- 37x12.50R1837.00 in tall+5.95 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Platforms recorded with this size
Full-size crossovers and half-ton pickups in their 18-inch packages.
Two notations, one height
31x10.50R15 and 31x10.50R16 both measure 31.00 in. That is not a calculation — in flotation notation the first number is the nominal overall diameter in inches, so a tire marked 31 is specified as a 31.
This size measures 31.05 in, and nothing on its sidewall says so. It has to be derived: an 18-inch wheel plus twice a sidewall that is 65% of 255 mm. The result is 0.16% above the flotation sizes, which is a difference of no practical consequence and a difference in how the information is presented that matters a great deal on a shelf.
| Size | Notation | Diameter | Section | Sidewall |
|---|---|---|---|---|
| 31x10.50R15 | flotation | 31.00 in | 10.50 in | 8.00 in |
| 31x10.50R16 | flotation | 31.00 in | 10.50 in | 7.50 in |
| 255/65R18 | metric | 31.05 in | 10.04 in | 6.53 in |
The two flotation entries are the same nominal tire on wheels an inch apart, so their sidewalls differ by exactly half an inch while their stated diameters do not differ at all. The metric vs imperial notation reference works through why the two systems answer different questions.
Its metric twin
255/70R17 measures 31.06 in — 0.01% from this size. Same casing width, one inch less wheel, five points more aspect ratio. The pair are interchangeable in every arithmetic sense.
Three neighbours, three different reasons
265/65R18 is 0.51 in taller at 31.56 in; 255/60R18 is 1.00 in shorter at 30.05 in; 285/60R18 is 0.41 in taller at 31.46 in on a much wider casing.
Section width against the flotation figures
The two flotation sizes above are marked 10.50 in wide and this one computes to 10.04 in — a gap of 0.46 in. Both numbers describe the same thing, the widest point of the inflated casing on a nominal rim, and both are equally subject to changing with rim width. The difference is that one is printed on the tire in inches and the other has to be converted from 255 mm.
Two constructions under one code
Both passenger-standard and LT casings are sold as 255/65R18, and their rated capacities are far apart. An LT-marked tire is not a bigger tire — the arithmetic on this page describes both equally — it is a heavier casing rated at a higher pressure. Which letters may appear in front of the size, and what each of them commits the manufacturer to, is set out in the tire construction codes reference.
Related
- 33x12.50R1833.00 in tall
- 35x12.50R1835.00 in tall
- 37x12.50R1837.00 in tall
Class: Crossover and SUV. Every figure above is computed from the size code, not stored — see how these numbers are calculated.