245/40R18 — dimensions, the 18-inch range by sidewall depth
A very common performance and aftermarket size, and a useful place to see that ordering tires by wheel size tells you nothing.
Metric notation · Performance and ultra-high performance
- Overall diameter
- 25.72 in
- 653 mm
- Section width
- 9.65 in
- 245 mm
- Sidewall height
- 3.86 in
- 98 mm
- Aspect ratio
- 40%
- sidewall as a share of width
- Wheel diameter
- 18 in
- the hole in the middle
- Circumference
- 80.8 in
- 2052 mm
- Revolutions per mile
- 784.2
- geometric, not measured
- Revolutions per km
- 487.3
- 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+7.28 in
- 35x12.50R1835.00 in tall+9.28 in
- 37x12.50R1837.00 in tall+11.28 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Platforms recorded with this size
Muscle cars, sports saloons and a very large aftermarket following. It is one of the most widely stocked 18-inch sizes in North America, and it turns up as an original fitment on Mustang, Camaro, Charger, and the mid-size German saloons.
The performance 18s, ordered by sidewall
| Size | Sidewall | Section | Diameter |
|---|---|---|---|
| 225/40R18 | 3.54 in | 8.86 in | 25.09 in |
| 235/40R18 | 3.70 in | 9.25 in | 25.40 in |
| 245/40R18 | 3.86 in | 9.65 in | 25.72 in |
| 225/45R18 | 3.99 in | 8.86 in | 25.97 in |
| 255/40R18 | 4.02 in | 10.04 in | 26.03 in |
| 235/45R18 | 4.16 in | 9.25 in | 26.33 in |
Sidewall depth and overall diameter move together here, but not in step: 225/45R18 has more sidewall than this size and only 0.26 in more diameter, while 255/40R18 has almost the same sidewall and 0.31 in more diameter. The two variables are independent and the code sets them jointly.
Its rolling-diameter twins
245/45R17 is 0.04 in below at 25.68 in and 245/35R19 is 0.04 in above at 25.75 in — the same 245 mm casing one inch of wheel either side, the classic three-rung stack. 205/60R16 sits 0.03 in below at 25.69 in, from an entirely different part of the market.
Speed symbols and the ZR marking
Tires in this size routinely carry W and Y speed symbols, and a proportion carry ZR in the size position — a hangover from a rating scheme in which Z marked the top of the scale rather than a specific figure. It occupies the construction slot in the code and changes nothing about the geometry above.
The current scheme is in the speed rating table; what the letters in the size field mean is in the tire construction codes reference.
Aftermarket wheels and where the tire ends up
A size this popular in the aftermarket meets a lot of wheels it was not delivered on. The two dimensions that decide whether the assembly clears the strut on the inside and the arch lip on the outside are offset and backspacing, and neither is in the tire code — a wheel of the correct diameter and width can still put this tire in the wrong place laterally. The wheel offset and backspacing reference sets out how the two are measured and how they differ.
Related
- 245/40R18 vs 255/35R19
- 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.