235/40R18
tyre size, drawn to scale
The dimensions
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·drawn to scale from the size designation — 1 px = 2.87 mm·REVIEWED ETRTO STANDARD
645 mm A 235/40R18 stands 645 mm — 25.40″ — from the ground to the top of the tread, on a sidewall of 94.0 mm.
Every one of those figures comes out of the designation itself: 235 mm across, a sidewall 40% of that, beaded onto a 18-inch wheel. It turns 493 times per kilometre, which is the number your speedometer and ABS are really counting.
- Overall diameter
- 645mm · 25.40″
- Sidewall height
- 94.0mm
- Section width
- 235mm · 9.25″
- Revolutions / km
- 493· 794/mi
How the figures are derived
- Read the size as the specification it is: section width in millimetres, aspect ratio as a percentage of that width, rim diameter in inches.
- Sidewall height = width × aspect ÷ 100. This is the number the size string is really about, and it is the one nobody quotes.
- Overall diameter = rim × 25.4 + two sidewalls — one at the top of the wheel and one at the bottom.
- Circumference = π × overall diameter, which gives revolutions per kilometre and per mile directly.
- Nothing above is measured, estimated, averaged or dated. It is the definition of the designation, so it does not go stale and it has no source to cite beyond the standard itself.
sidewall (mm) = width × aspect ÷ 100 overall diameter (mm) = rim × 25.4 + 2 × sidewall speedo true speed = indicated × (new diameter ÷ old diameter)
The full method, the ±3% rule, and what geometry cannot tell you →
What the three numbers actually say
The size on your sidewall is a specification, and each field of it means one thing.
- 235
- Section width, 235 millimetres. The tyre's width at its widest point when it is mounted and inflated — sidewall bulge included, not the tread. In inches that is 9.25″.
- 40
- Aspect ratio, 40%. The sidewall is 40% as tall as the tyre is wide, so 235 × 40 ÷ 100 = 94.0 mm. A lower number is a shorter sidewall and a stiffer ride.
- R18
- Radial construction, 18-inch rim. The R is the carcass type, not a size. The 18 is the wheel diameter the tyre beads onto — 457.2 mm — and it is the one dimension of the three that is still in inches.
The load index and speed symbol that follow the size — the 91V part — are a separate code. Read the rest of the sidewall →
Full dimensions
| Overall diameter | 645.2 mm | 25.40″ |
|---|---|---|
| Sidewall height | 94.0 mm | 3.70″ |
| Section width | 235 mm | 9.25″ |
| Rim diameter | 18″ | 457.2 mm |
| Rolling circumference | 2027 mm | 2.027 m |
| Revolutions per kilometre | 493.4 | 794.0 per mile |
| Ride height contribution | 322.6 mm | hub centre to ground, unloaded |
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·computed from the 235/40R18 designation — no catalogue, no measurement, no vintage·REVIEWED ETRTO STANDARD
What else fits where a 235/40R18 does
Every size within reach of 645 mm, closest first, judged against the ±3% convention.
| Alternative | Diameter | Change | Verdict | Speedo at 100 |
|---|---|---|---|---|
| 225/40R18 | 637 mm | −1.24% | Within tolerance | 98.8 km/h true |
| 245/40R18 | 653 mm | +1.24% | Within tolerance | 101.2 km/h true |
| 235/35R19 | 647 mm | +0.29% | Within tolerance | 100.3 km/h true |
| 235/45R17 | 643 mm | −0.29% | Within tolerance | 99.7 km/h true |
| 215/45R18 | 651 mm | +0.85% | Within tolerance | 100.9 km/h true |
| 265/35R18 | 643 mm | −0.39% | Within tolerance | 99.6 km/h true |
| 205/45R18 | 642 mm | −0.54% | Within tolerance | 99.5 km/h true |
| 255/35R18 | 636 mm | −1.47% | Within tolerance | 98.5 km/h true |
Ranked by how close each one holds the original rolling diameter — which is the only thing this table can rank on. Whether any of them clears your arches, suits your rim width or carries your car's load is decided by the placard on your door pillar, not by this list.
What each one does to the speedometer
Assuming the car was calibrated for 235/40R18 and you fit the alternative instead. Indicated 100 km/h in every row.
| Fitted instead | Diameter change | Speedo shows 100 | True speed | Reads |
|---|---|---|---|---|
| 225/40R18 | −1.24% | 100 km/h | 98.8 km/h | high — you are slower than shown |
| 245/40R18 | +1.24% | 100 km/h | 101.2 km/h | low — you are faster than shown |
| 235/35R19 | +0.29% | 100 km/h | 100.3 km/h | low — you are faster than shown |
Reading low is the direction with no legal tolerance in it. Why that is, and what the rule says where you live →
235/40R18 — the questions that follow
What is the overall diameter of a 235/40R18 tyre?
645 mm, or 25.40″. That is the 18-inch rim (457.2 mm) plus two sidewalls of 94.0 mm each — one above the wheel centre and one below.
How tall is the sidewall on a 235/40R18?
94.0 mm. The 40 in the size is a percentage, not a measurement: the sidewall is 40% of the 235 mm section width, so 235 × 40 ÷ 100 = 94.0 mm.
What can I fit instead of a 235/40R18?
The closest alternatives on rolling diameter are 225/40R18 (−1.24%), 245/40R18 (+1.24%), 235/35R19 (+0.29%). Anything within ±3% of 645 mm keeps the speedometer and stability systems inside the band they were calibrated for — but arch clearance, rim width and load rating are separate questions that a size string cannot answer.
How many times does a 235/40R18 turn in a kilometre?
493 revolutions per kilometre, or 794 per mile. Its rolling circumference is 2.027 m, which is what the speedometer and the ABS both count against.
Neighbouring sizes on a 18-inch rim
The geometry here is exact; whether a tyre fits YOUR car is not something geometry can answer. Arch clearance, suspension travel, offset, load rating and your vehicle's own placard all decide that, and none of them are in a size string. The ±3% rule quoted throughout is an industry convention, not a law. Confirm any change against the tyre placard on your door pillar and with a tyre fitter before you buy.