315/70R18 tire size
35.36 inches tall, 12.4 inches wide, on an 18 inch rim. Sidewall 8.68 in (220.5 mm) per side, 570 revolutions per mile.
This is geometry, not fitment approval. It says what a size measures, never whether it fits a vehicle.
- Overall diameter
- 35.36 in
- Sidewall
- 220.5 mm
- Revs / mile
- 570
315/70R18 in numbers
315/70R18 works out at 35.36 inches (898 mm) tall and 315 mm (12.4 in) wide, with 220.5 mm of sidewall on each side of an 18 inch rim. One turn covers 2822 mm (111.1 in), so it turns about 354 times per kilometre and 570 times per mile, or 591 times a minute at 100 km/h. Of the 555 passenger sizes listed here, 5 are taller and 549 are shorter. The light truck version, LT315/70R18, shares these dimensions and is rated on the LT load table instead. 24 other passenger sizes come within 3 percent of its diameter; the closest is 325/60R20 (-0.02 percent).
What each part of 315/70R18 means
- 315
- Section width in millimetres, sidewall to sidewall at the widest point. Not the tread width, which is narrower.
- 70
- Aspect ratio. The sidewall is 70 percent of the section width: 315 x 70 / 100 = 220.5 mm. A lower number is a shorter sidewall on the same width.
- R
- Radial construction. The R here is construction, not the speed symbol R. The speed symbol sits after the size, in the service description.
- 18
- Rim diameter in inches. The bead seat diameter of the wheel this tire mounts on.
- no prefix
- Euro-metric (no prefix). An unprefixed size is Euro-metric, not P-metric with the P left off. The prefix decides which standards body's load table rates the tire. What the prefixes mean
Dimensions 315/70R18 implies
| Dimension | Imperial | Metric | Where it comes from |
|---|---|---|---|
| Section width | 12.4 in | 315 mm | Printed on the tire in millimetres. Nominal: a real tire measures wider on a wide rim and narrower on a narrow one. |
| Sidewall height (one side) | 8.68 in | 220.5 mm | Computed. Bead seat to tread, one side only. |
| Overall diameter | 35.36 in | 898.2 mm | Computed from rim plus two sidewalls. |
| Rim diameter | 18 in | 457.2 mm | Printed on the tire. The bead seat diameter of the wheel it mounts on. |
| Circumference | 111.09 in | 2821.8 mm | Computed as pi times the nominal overall diameter. |
| Revolutions | 570.3 per mile | 354.4 per km | Computed from the nominal circumference. A loaded tire turns slightly more often. |
These are the nominal dimensions the size code implies. A real tire's measured dimensions depend on its mould, its construction and the rim width it is mounted on, so a manufacturer's specification sheet for a specific tire is the authority for that tire.
The overall diameter formula used here reproduces all 52 radial passenger design diameters published in an Indian vehicle standard to within 1.4 mm, 0.5 mm on average: AIS-044 (Part 2) Amendment 2, Annexure A.
Sizes within 3 percent of 315/70R18, any rim
The 10 sizes in this catalog closest to 315/70R18 in overall diameter, of the same type, ranked by how little they change it. The speedometer columns assume the instrument read correctly on 315/70R18 and that nothing was recalibrated.
| Size | Rim | Overall diameter | Difference | True speed at an indicated 100 km/h | True speed at an indicated 60 mph |
|---|---|---|---|---|---|
| 325/60R20 | 20 in | 35.35 in (898 mm) | -0.02% | 100 km/h (0.0) | 60 mph (0.0) |
| 325/45R24 | 24 in | 35.52 in (902 mm) | +0.43% | 100.4 km/h (+0.4) | 60.3 mph (+0.3) |
| 305/55R22 | 22 in | 35.21 in (894 mm) | -0.43% | 99.6 km/h (-0.4) | 59.7 mph (-0.3) |
| 355/40R24 | 24 in | 35.18 in (894 mm) | -0.51% | 99.5 km/h (-0.5) | 59.7 mph (-0.3) |
| 315/45R24 | 24 in | 35.16 in (893 mm) | -0.57% | 99.4 km/h (-0.6) | 59.7 mph (-0.3) |
| 295/65R20 | 20 in | 35.1 in (891 mm) | -0.75% | 99.3 km/h (-0.7) | 59.6 mph (-0.4) |
| 285/70R20 | 20 in | 35.71 in (907 mm) | +0.98% | 101 km/h (+1.0) | 60.6 mph (+0.6) |
| 275/60R22 | 22 in | 34.99 in (889 mm) | -1.05% | 99 km/h (-1.0) | 59.4 mph (-0.6) |
| 315/60R20 | 20 in | 34.88 in (886 mm) | -1.36% | 98.6 km/h (-1.4) | 59.2 mph (-0.8) |
| 285/75R18 | 18 in | 34.83 in (885 mm) | -1.50% | 98.5 km/h (-1.5) | 59.1 mph (-0.9) |
A positive difference means the replacement is taller, so the vehicle travels further per revolution and the speedometer reads LOW. Many vehicles leave the factory reading slightly high on purpose, so treat this as a change from whatever the instrument did before, not as an absolute claim about it.
Minus one: 17 inch rim, same rolling diameter
A smaller wheel with a taller sidewall. Common for winter fitments, where sidewall is wanted and wheel diameter is not.
| Size | Overall diameter | Difference | True speed at an indicated 100 km/h | True speed at an indicated 60 mph |
|---|---|---|---|---|
| 295/75R17 | 34.42 in (874 mm) | -2.66% | 97.3 km/h (-2.7) | 58.4 mph (-1.6) |
| 315/70R17 | 34.36 in (873 mm) | -2.83% | 97.2 km/h (-2.8) | 58.3 mph (-1.7) |
| 275/80R17 | 34.32 in (872 mm) | -2.94% | 97.1 km/h (-2.9) | 58.2 mph (-1.8) |
A positive difference means the replacement is taller, so the vehicle travels further per revolution and the speedometer reads LOW. Many vehicles leave the factory reading slightly high on purpose, so treat this as a change from whatever the instrument did before, not as an absolute claim about it.
Same 315 mm width, different aspect ratio
What changing only the aspect ratio does, holding the width and the rim.
Questions this size raises
- What does 315/70R18 mean?
- 315 is the section width in millimetres. 70 is the aspect ratio: the sidewall is 70 percent of the section width, which works out at 220.5 mm. Radial construction. 18 is the rim diameter in inches. The service type is Euro-metric (no prefix).
- How tall is a 315/70R18 tire?
- 35.36 inches (898.2 mm) in nominal overall diameter. That is the diameter the size code implies; a specific tire's measured diameter depends on its mould and the rim width it is mounted on, and the manufacturer's specification sheet is the authority for that tire.
- What is the sidewall height of a 315/70R18?
- 8.68 inches (220.5 mm) per side.
- How many revolutions per mile does a 315/70R18 turn?
- 570 revolutions per mile, from the nominal circumference of 111.1 inches. A loaded tire deflects and turns slightly more often than that, so a manufacturer's published revolutions figure usually runs a few percent higher.
- What is the minus one size for 315/70R18?
- 295/75R17 keeps the overall diameter closest on a rim one inch smaller, of the sizes in this catalog. Minus sizing is common for winter fitments, where a taller sidewall and a narrower tread are wanted.
The rest of the sidewall
The size is one line of a sidewall. What follows it decides whether the tire is legal on the vehicle at all.
- Load index table - the number after the size, and the mass it stands for. Must meet or exceed the vehicle placard.
- Speed symbol table - the letter after the load index, including the ZR, W, Y and (Y) cases.
- DOT date code - how old the tire is, and why a three digit code is not the same as a four digit one.
- Every other marking - UTQG grades, load range letters, M+S against the three peak mountain snowflake.
- Compare 315/70R18 against another size - diameter, clearance and speedometer error side by side.
Where these figures come from
Computed on this page, not looked up. Each figure and the arithmetic behind it:
- Sidewall height (one side)
section width x aspect ratio / 100 = 315 x 70 / 100 = 220.5 mm - Overall diameter
rim diameter + 2 x sidewall = (18 x 25.4) + 2 x 220.5 = 457.2 + 441 = 898.2 mm = 35.362 in - Circumference
pi x overall diameter = pi x 898.2 = 2821.8 mm = 111.09 in - Revolutions per mile
63360 inches per mile / circumference in inches = 63360 / 111.09 = 570.3 - Revolutions per kilometre
1000000 mm per km / circumference in mm = 1000000 / 2821.8 = 354.4
Published values here were read from 2 independent sources:
- Published tire size sitemap - SimpleTire, sitemap dated 13 August 2026 (read 2026-08-27)Which sizes a national retailer publishes a catalog page for
- Published tire size listing sitemap - Discount Tire, sitemap read 27 August 2026 (read 2026-08-27)Which sizes a second national retailer publishes a catalog page for
These are independent publishers, not independent measurements. Each one reproduces a table originally set by a standards body, so agreement between them shows a value was transcribed correctly, not that the underlying table is right. Where a figure is safety relevant, check it against the tire manufacturer's own data before relying on it.
Before you fit anything
SidewallRef calculates the dimensions a size code implies and reproduces the published load index and speed symbol tables. It does not know your vehicle.
Load index and speed rating must meet or exceed what the vehicle placard specifies. The placard is in the driver's door jamb on most vehicles and is the authority for your vehicle, not this site.
Clearance at full lock and full suspension travel, wheel width and offset, brake and suspension component clearance, axle load capacity, inflation pressure and any legal requirement where you drive are the owner's and the fitter's responsibility.
Nothing here approves a fitment. Confirm any change with a qualified tire fitter and with the vehicle manufacturer's own specification before driving on it.