Compare two sizes › Tire sizes › 245/55R18
245/55R18: 726.7 mm (28.61 in) tall
A 245/55R18 tire is 245 mm wide with a 134.75 mm sidewall on an 18-inch rim. It turns 704.9 times per mile. Below: every dimension, a drawing to scale, and the 23 sizes within ±3 % of its diameter on 17, 18 and 19 in rims, with what the speedometer would read on each.
Every dimension
| Measure | mm | inches |
|---|---|---|
| Section widthsidewall to sidewall, as printed | 245.0 mm | 9.65 in |
| Sidewall height55 % of the width | 134.75 mm | 5.31 in |
| Rim diameterbead seat, as printed | 457.2 mm | 18 in |
| Overall diameterrim + two sidewalls | 726.7 mm | 28.61 in |
| Circumferenceπ × diameter | 2,283.0 mm | 89.88 in |
| Revolutions per mileunloaded, geometric | 704.9 | |
| Revolutions per kmunloaded, geometric | 438.0 | |
Nominal figures for a new, unloaded tire. Under a car's weight it squashes and turns roughly 2–3 % more often per mile; the maker's datasheet figure is measured and wins.
What 245/55R18 means
- 245
- Section width in millimetres (9.65 in), sidewall to sidewall. It is measured on the rim width the tire standard lists for that size; on a wider or narrower rim it changes by roughly 5 mm per half inch.
- 55
- Aspect ratio: the sidewall is 55 % as tall as the tire is wide — 245 × 55 ÷ 100 = 134.75 mm. It is a percentage, not a height.
- R
- Radial construction (not "radius"). Written ZR, it is a radial rated above 240 km/h.
- 18
- Rim diameter in inches (457.2 mm): the wheel this tire mounts on.
- after
- The load index and speed symbol follow the size (for example 245/55R18 91V or 94W XL). The same size is sold with several; type yours into the calculator below to decode it.
Drawn to scale
Closest swap
On the same rim, 225/60R18 is nearest in diameter: +0.1 %. Going up to a 19-inch rim, 245/50R19 is nearest: +0.1 %, so the speedometer reads −0.1 %; at an indicated 60 mph you would do 60.1 mph.
Sizes within ±3 % of 245/55R18
Every size in the grid (widths 225–265 mm in 10 mm steps, aspect ratios 25–80 in steps of 5) whose overall diameter is within ±3 % of 726.7 mm. ±3 % is a rule of thumb many tire fitters use, not a law. The “really doing” column assumes the speedometer was right on 245/55R18: it is your true speed when the speedometer shows 60 mph (and, below it, 100 km/h). Not every size here is made by every maker, and a size being close in diameter says nothing about clearance, load rating or what the car maker approves. Tap a size to load it into the calculator.
Same 18-inch rim 6
| Size | Diameter | vs 245/55R18 | Really doing |
|---|---|---|---|
| 225/60R18 | 727.2 mm28.63 in | +0.1 %+0.5 mm | 60.0 mph100.1 km/h |
| 265/50R18 | 722.2 mm28.43 in | −0.6 %−4.5 mm | 59.6 mph99.4 km/h |
| 235/55R18 | 715.7 mm28.18 in | −1.5 %−11.0 mm | 59.1 mph98.5 km/h |
| 255/55R18 | 737.7 mm29.04 in | +1.5 %+11.0 mm | 60.9 mph101.5 km/h |
| 235/60R18 | 739.2 mm29.10 in | +1.7 %+12.5 mm | 61.0 mph101.7 km/h |
| 255/50R18 | 712.2 mm28.04 in | −2.0 %−14.5 mm | 58.8 mph98.0 km/h |
One inch smaller: 17-inch rim 8
| Size | Diameter | vs 245/55R18 | Really doing |
|---|---|---|---|
| 245/60R17 | 725.8 mm28.57 in | −0.1 %−0.9 mm | 59.9 mph99.9 km/h |
| 225/65R17 | 724.3 mm28.52 in | −0.3 %−2.4 mm | 59.8 mph99.7 km/h |
| 265/55R17 | 723.3 mm28.48 in | −0.5 %−3.4 mm | 59.7 mph99.5 km/h |
| 235/65R17 | 737.3 mm29.03 in | +1.5 %+10.6 mm | 60.9 mph101.5 km/h |
| 255/60R17 | 737.8 mm29.05 in | +1.5 %+11.1 mm | 60.9 mph101.5 km/h |
| 235/60R17 | 713.8 mm28.10 in | −1.8 %−12.9 mm | 58.9 mph98.2 km/h |
| 255/55R17 | 712.3 mm28.04 in | −2.0 %−14.4 mm | 58.8 mph98.0 km/h |
| 225/70R17 | 746.8 mm29.40 in | +2.8 %+20.1 mm | 61.7 mph102.8 km/h |
Plus one: 19-inch rim 9
| Size | Diameter | vs 245/55R18 | Really doing |
|---|---|---|---|
| 245/50R19 | 727.6 mm28.65 in | +0.1 %+0.9 mm | 60.1 mph100.1 km/h |
| 225/55R19 | 730.1 mm28.74 in | +0.5 %+3.4 mm | 60.3 mph100.5 km/h |
| 265/45R19 | 721.1 mm28.39 in | −0.8 %−5.6 mm | 59.5 mph99.2 km/h |
| 235/50R19 | 717.6 mm28.25 in | −1.3 %−9.1 mm | 59.2 mph98.7 km/h |
| 255/50R19 | 737.6 mm29.04 in | +1.5 %+10.9 mm | 60.9 mph101.5 km/h |
| 235/55R19 | 741.1 mm29.18 in | +2.0 %+14.4 mm | 61.2 mph102.0 km/h |
| 255/45R19 | 712.1 mm28.04 in | −2.0 %−14.6 mm | 58.8 mph98.0 km/h |
| 225/50R19 | 707.6 mm27.86 in | −2.6 %−19.1 mm | 58.4 mph97.4 km/h |
| 265/50R19 | 747.6 mm29.43 in | +2.9 %+20.9 mm | 61.7 mph102.9 km/h |
Compare 245/55R18 with any size
Working it out…
Speedometer error
The speedometer counts wheel turns and assumes the current tire.
| Speedometer shows | You are really doing | Difference |
|---|
Both to scale
What the codes say
Load index and speed rating tables (ISO 4000-1 / ETRTO)
Side by side
| Measure | Current | New | Difference |
|---|
The sums for 245/55R18
sidewall = 245 × 55 ÷ 100 = 134.75 mm diameter = 18 × 25.4 + 2 × 134.75 = 726.7 mm circumference = π × 726.7 = 2,283.0 mm revs per mile = 1,609,344 ÷ 2,283.0 = 704.9 revs per km = 1,000,000 ÷ 2,283.0 = 438.0
Speedometer error is the ratio of the two diameters: true speed = shown speed × new diameter ÷ 245/55R18 diameter. Fitting 245/50R19 (727.6 mm) in place of 245/55R18, an indicated 100 km/h is really 100.1 km/h, and the odometer is off by the same proportion.
Why this size has a page
We found no public list of the most-sold tire sizes that we are allowed to reuse, so the 5 sizes on this ladder follow the same plus-sizing rule as the other ladders. 225/65R17 is the anchor for crossover-style sizes: a tall 65-series sidewall on a 17-inch rim. For each rim from 16 to 20 inches, pick the size with the closest overall diameter to 225/65R17 among widths 205 + 10 × (rim − 17) to 245 + 10 × (rim − 17) mm in 10 mm steps and aspect ratios 40–75 in steps of 5. 245/55R18 is on that ladder, 2.4 mm from 225/65R17's 724.3 mm diameter. From the 16-inch step (225/70R16) to the 20-inch step (275/40R20) the sidewall drops from 157.5 to 110.0 mm while the diameter stays within 3.7 mm. All size pages and their rules.
What this page cannot tell you
- Whether a size is right for your car. Clearance to struts, arches and brake calipers, wheel width and offset, steering lock and suspension travel decide that, and none of them is in the size code. Start from the sizes the car maker lists (door-jamb placard, handbook, registration papers), then check the physical clearance.
- Load capacity. An alternative size must carry at least the load index and speed rating the car maker specifies; a narrower or lower-profile tire often has a lower load index in the same construction, so check the XL / reinforced version.
- Exact real-world size. Makers build to a tolerance of a few percent, and a new tire with 8 mm of tread is about 13 mm taller than one worn to 1.6 mm.
- Your speedometer's own error. Under UN ECE Regulation 39 a speedometer must never read below true speed and may read up to 10 % + 4 km/h high, so most cars already over-read a little. A taller tire eats into that margin.