345/40R15
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
657 mm A 345/40R15 stands 657 mm — 25.87″ — from the ground to the top of the tread, on a sidewall of 138.0 mm.
Every one of those figures comes out of the designation itself: 345 mm across, a sidewall 40% of that, beaded onto a 15-inch wheel. It turns 484 times per kilometre, which is the number your speedometer and ABS are really counting.
- Overall diameter
- 657mm · 25.87″
- Sidewall height
- 138.0mm
- Section width
- 345mm · 13.58″
- Revolutions / km
- 484· 780/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.
- 345
- Section width, 345 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 13.58″.
- 40
- Aspect ratio, 40%. The sidewall is 40% as tall as the tyre is wide, so 345 × 40 ÷ 100 = 138.0 mm. A lower number is a shorter sidewall and a stiffer ride.
- R15
- Radial construction, 15-inch rim. The R is the carcass type, not a size. The 15 is the wheel diameter the tyre beads onto — 381.0 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 | 657.0 mm | 25.87″ |
|---|---|---|
| Sidewall height | 138.0 mm | 5.43″ |
| Section width | 345 mm | 13.58″ |
| Rim diameter | 15″ | 381.0 mm |
| Rolling circumference | 2064 mm | 2.064 m |
| Revolutions per kilometre | 484.5 | 779.7 per mile |
| Ride height contribution | 328.5 mm | hub centre to ground, unloaded |
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·computed from the 345/40R15 designation — no catalogue, no measurement, no vintage·REVIEWED ETRTO STANDARD
345/40R15 — the questions that follow
What is the overall diameter of a 345/40R15 tyre?
657 mm, or 25.87″. That is the 15-inch rim (381.0 mm) plus two sidewalls of 138.0 mm each — one above the wheel centre and one below.
How tall is the sidewall on a 345/40R15?
138.0 mm. The 40 in the size is a percentage, not a measurement: the sidewall is 40% of the 345 mm section width, so 345 × 40 ÷ 100 = 138.0 mm.
What can I fit instead of a 345/40R15?
This size sits at the edge of the conventional fitment envelope, so there are no close alternatives inside the ±3% band on this rim. Check your vehicle's tyre placard before considering anything wider or taller.
How many times does a 345/40R15 turn in a kilometre?
484 revolutions per kilometre, or 780 per mile. Its rolling circumference is 2.064 m, which is what the speedometer and the ABS both count against.
Neighbouring sizes on a 15-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.