225/45R18 tire size — three codes, one rolling diameter
A premium-compact fitment that shares its height with two performance sizes it has nothing else in common with.
Metric notation · Performance and ultra-high performance
- Overall diameter
- 25.97 in
- 660 mm
- Section width
- 8.86 in
- 225 mm
- Sidewall height
- 3.99 in
- 101 mm
- Aspect ratio
- 45%
- sidewall as a share of width
- Wheel diameter
- 18 in
- the hole in the middle
- Circumference
- 81.6 in
- 2073 mm
- Revolutions per mile
- 776.5
- geometric, not measured
- Revolutions per km
- 482.5
- 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+7.03 in
- 35x12.50R1835.00 in tall+9.03 in
- 37x12.50R1837.00 in tall+11.03 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Premium compact saloons and hatchbacks account for most of the fitment: 3-series and A4 in base trim, A3 and 1-series in upper trims, Golf and Focus in their sportier specifications.
A three-way near-tie
255/40R18 measures 26.03 in. 255/35R19 measures 26.03 in. This size measures 25.97 in. All three are inside a quarter of one per cent of each other.
What separates them is the shape of the casing, not its size:
| Size | Wheel | Section | Sidewall |
|---|---|---|---|
| 225/45R18 | 18 in | 8.86 in | 3.99 in |
| 255/40R18 | 18 in | 10.04 in | 4.02 in |
| 255/35R19 | 19 in | 10.04 in | 3.51 in |
The 255/40R18 pairing is the interesting one: same wheel, 1.18 in more section width, and essentially the same sidewall depth and standing height. It is the front-and-rear pairing of a staggered set expressed as two entries in a catalogue, and the staggered fitments reference covers what running two different sizes on one vehicle involves.
Same width, one wheel size down
225/50R17 measures 25.86 in — 0.11 in below this size, 0.44%. Both carry 8.86 in of casing across the tread; the 17-inch code makes up the inch of missing wheel with five extra points of profile, and lands a ninth of an inch short. That is the plus-one relationship in its textbook form, and it is why both sizes turn up on the same model in different trims.
Its own wheel, at 45-series
235/45R18 is 0.35 in taller at 26.33 in, 245/45R18 is 0.71 in taller at 26.68 in, and 255/45R18 is 1.06 in taller at 27.04 in. Every 10 mm adds 0.35 in, as it does at 45-series on any wheel.
Wheels, not just tires
Because so many cars on this size run aftermarket wheels, the dimension that most often causes trouble is not in the tire code at all. Offset and backspacing decide where the assembly sits relative to the strut and the arch lip, and two 18-inch wheels of the same width can place the same tire an inch apart laterally. The wheel offset and backspacing reference explains how the two measurements relate.
Related
- 33x12.50R1833.00 in tall
- 35x12.50R1835.00 in tall
- 37x12.50R1837.00 in tall
Class: Performance and ultra-high performance. Every figure above is computed from the size code, not stored — see how these numbers are calculated.