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Tire sizes

255/45R18 — the widest 45-series on an 18-inch wheel

A wide, comparatively deep-sidewall performance fitment — and a good place to say what eight characters cannot tell you.

Metric notation · Performance and ultra-high performance

Overall diameter
27.04 in
687 mm
Section width
10.04 in
255 mm
Sidewall height
4.52 in
115 mm
Aspect ratio
45%
sidewall as a share of width
Wheel diameter
18 in
the hole in the middle
Circumference
84.9 in
2157 mm
Revolutions per mile
746.0
geometric, not measured
Revolutions per km
463.5
geometric, not measured
Typical load index
88–104
1235–1984 lb per tire

Low-profile sizes hold less air for their width, so the same vehicle weight needs a higher reference inflation. Extra Load construction is common in this class, and that is what lifts the index band — not any change in the casing size.

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+5.96 in
  • 35x12.50R1835.00 in tall+7.96 in
  • 37x12.50R1837.00 in tall+9.96 in

Compare any two sizes, including ones this site has no page for, in the comparison tool.

Performance saloons, coupes and sporting estates, and a common rear size where the front runs narrower. Nothing at 45-series on an 18-inch wheel in this catalogue is wider.

What the code fixes

Everything above. Overall diameter, section width, sidewall depth, circumference and the geometric revolutions-per-mile figure all follow from the eight characters by arithmetic, and two tires carrying this code will agree on all of them to within manufacturing tolerance.

What it does not fix

Tread compound, tread pattern and therefore grip in any condition. Treadwear, traction and temperature grades, which are recorded in the UTQG marking and not in the size — and the treadwear number in particular is assigned by the manufacturer against its own reference, so comparing it across brands is not meaningful. The UTQG ratings reference explains what the grades do and do not measure.

Load index and speed symbol, which follow the size in the service description and vary widely within it. Whether the casing is reinforced. Whether the tire carries the three-peak snowflake marking, which is a severe-snow certification and not a size property — the M+S and 3PMSF markings reference covers the difference between that and the older M+S stamp.

And the actual measured width, which depends on the rim it is mounted to.

The 255 mm sizes under 28 inches

Size Wheel Sidewall Diameter
255/35R19 19 in 3.51 in 26.03 in
255/40R18 18 in 4.02 in 26.03 in
255/35R20 20 in 3.51 in 27.03 in
255/40R19 19 in 4.02 in 27.03 in
255/45R18 18 in 4.52 in 27.04 in

Five entries, two rolling diameters, one section width. This size has the most sidewall of any of them, which on a 255 mm casing is 4.52 in.

Widening at a fixed aspect ratio and wheel raises the tire as well: at 45-series each 10 mm adds 0.35 in of height, so 255/45R18 stands 1.06 in above 225/45R18 rather than merely sitting wider than it. A wider tire is never only a wider tire.

Related

Class: Performance and ultra-high performance. Every figure above is computed from the size code, not stored — see how these numbers are calculated.