A 315 tire is about 12.4 inches wide, because the first number in a metric tire size gives its nominal section width in millimeters. That number alone does not tell the tire’s full height or the wheel size it fits. For those details, the complete size—such as 315/70R17—is needed. Understanding each part helps drivers compare tires, check fitment, and avoid confusing width with overall diameter.
What Does 315 Mean on a Tire?
In a metric tire size such as 315/70R17, 315 indicates the tire’s nominal section width: the approximate distance from one sidewall to the other at the tire’s widest point when mounted on a specified measuring rim and inflated under prescribed conditions. The measurement is in millimeters, not inches.
To convert millimeters to inches, divide by 25.4. The calculation is 315 ÷ 25.4 = 12.40, so a 315 tire is commonly described as approximately 12.4 inches wide.
This is a nominal measurement, not a guarantee that every tire marked 315 will measure exactly 12.4 inches on a vehicle. Actual dimensions can vary by tire model, tread design, measuring-rim width, inflation pressure, and measurement method. The tire manufacturer’s specifications provide the best estimate for a particular product.
A 315 Tire Is Not Necessarily 315 Inches in Diameter
The 315 figure describes width, not height. A tire’s overall diameter depends on three parts of its complete size: section width, aspect ratio, and wheel diameter. For example, 315/70R17 and 315/35R20 both have a nominal section width of 315 millimeters, but their sidewalls and overall diameters are quite different.
The complete size also identifies the wheel diameter the tire is designed to fit. A 315/70R17 tire is made for a 17-inch wheel, while a 315/35R20 is made for a 20-inch wheel. The wheel diameter is the last number in the size, not the tire’s overall diameter.
How to Read a 315 Tire Size
Consider the example 315/70R17. Each part communicates a different dimension or construction detail:
- 315: Nominal section width in millimeters, or about 12.4 inches.
- 70: Aspect ratio. The sidewall height is 70% of the nominal section width.
- R: Radial construction, the standard construction type for most passenger and light-truck tires.
- 17: Wheel diameter in inches. The tire must fit a 17-inch wheel.
Some light-truck tires include a prefix such as LT, as in LT315/70R17. The LT designation identifies a tire intended for light-truck applications; it does not change the meaning of 315. Load range, load index, and speed rating may also appear elsewhere on the sidewall and should be considered when selecting a replacement.
How to Calculate a 315 Tire’s Approximate Diameter
A tire’s theoretical overall diameter can be estimated from its metric size. First, find the sidewall height by multiplying the section width by the aspect ratio. Convert that height from millimeters to inches, double it to account for both sidewalls, and add the wheel diameter.
Approximate diameter in inches = [2 × (section width in millimeters × aspect ratio ÷ 100) ÷ 25.4] + wheel diameter in inches.
For a 315/70R17, the sidewall calculation is 315 × 0.70 = 220.5 millimeters, or about 8.68 inches. Two sidewalls measure about 17.36 inches. Add the 17-inch wheel diameter, and the calculated overall diameter is about 34.36 inches.
This is a mathematical estimate based on nominal dimensions. A tire’s published specifications may show a slightly different diameter, and its measured diameter can change with the rim used, inflation, load, tread depth, and manufacturing design. For fitment decisions, use the specific tire maker’s data rather than relying on the formula alone.
Common 315 Tire Sizes and Approximate Dimensions
The table shows how different aspect ratios and wheel diameters change a 315 tire’s estimated sidewall height and overall diameter. The width remains approximately 12.4 inches in each example.
| Example Size | Approx. Width | Approx. Sidewall Height | Approx. Overall Diameter | Wheel Diameter |
|---|---|---|---|---|
| 315/35R20 | 12.4 in | 4.3 in | 28.7 in | 20 in |
| 315/60R20 | 12.4 in | 7.4 in | 34.9 in | 20 in |
| 315/70R17 | 12.4 in | 8.7 in | 34.4 in | 17 in |
| 315/75R16 | 12.4 in | 9.3 in | 34.6 in | 16 in |
These figures are calculated estimates, not guaranteed measurements for every product carrying the listed size. Some size combinations may be uncommon or unavailable for a particular tire category. Always confirm that a specific tire is offered in the required size and is approved for the vehicle’s wheel and load requirements.
Why the Aspect Ratio Matters
The aspect ratio is the second number in a metric tire size. It expresses sidewall height as a percentage of section width. A 315/70 tire has a nominal sidewall height equal to 70% of 315 millimeters, while a 315/35 has a sidewall height equal to 35% of that width.
Because the width is the same, a higher aspect ratio generally means a taller sidewall. But overall tire diameter also depends on wheel diameter. A low-profile tire on a larger wheel can end up close in overall diameter to a taller-sidewall tire on a smaller wheel, even though the two look different and may handle differently.
Sidewall height can affect ride characteristics, wheel protection, and how a tire responds to steering inputs. Those effects depend on the whole tire and vehicle setup; the aspect ratio alone does not determine comfort or performance.
How a 315 Tire Compares With Other Widths
Since the width is stated in millimeters, converting other common metric widths can make comparisons easier. The inch figures below are approximate nominal widths:
- 285 tire: 285 ÷ 25.4 = about 11.2 inches.
- 295 tire: 295 ÷ 25.4 = about 11.6 inches.
- 305 tire: 305 ÷ 25.4 = about 12.0 inches.
- 315 tire: 315 ÷ 25.4 = about 12.4 inches.
- 325 tire: 325 ÷ 25.4 = about 12.8 inches.
The difference between two nominal widths does not always equal the difference between their measured widths on a vehicle. A 315 tire is nominally 10 millimeters wider than a 305 tire—about 0.39 inch—but the actual difference can vary across products and wheel widths.
Will a 315 Tire Fit a Vehicle?
Width alone cannot determine fitment. A 315 tire may fit one vehicle and rub or sit incorrectly on another. Before changing sizes, drivers need to check the vehicle manufacturer’s approved tire sizes, the wheel’s diameter and width, wheel offset, suspension and body clearance, and the tire’s load capacity.
A wider tire can have less clearance to suspension components, fenders, or other parts, particularly while turning or when the suspension compresses. Fit can also be affected by wheel offset and changes to suspension height. A tire that appears to clear while parked may contact the vehicle during steering or under load.
Changing overall diameter can affect ground clearance, speedometer readings, gearing, and the operation of systems that rely on wheel-speed information. When tire diameters differ substantially from the original equipment size, the indicated vehicle speed may no longer match actual speed. The exact effect depends on the vehicle and the size change.
The vehicle’s tire placard—often located on the driver-side doorjamb—and owner’s manual list the manufacturer’s original tire specifications and recommended cold inflation pressure. A tire professional can also verify wheel compatibility, load capacity, and clearance before installation.
Width, Load Capacity, and Inflation Pressure
A tire’s width does not by itself indicate how much weight it can carry. Load capacity depends on the tire’s load index or load range, construction, and inflation pressure, among other specifications. A replacement tire should meet the vehicle manufacturer’s load requirements; a wider size is not automatically a suitable or safer upgrade.
Inflation pressure should be set according to the vehicle manufacturer’s recommendation for the approved setup, unless a qualified tire professional or vehicle manufacturer specifies otherwise. The maximum pressure printed on a tire sidewall is not the normal target pressure for everyday driving. Drivers should check pressure when tires are cold and use a reliable gauge.
How to Check a Tire’s Actual Specifications
For a precise answer about a particular 315 tire, identify its complete sidewall size and manufacturer. Then check the manufacturer’s product page or specification sheet for listed section width, overall diameter, approved rim-width range, load index, and load range. These details are more useful than the 315 marking by itself.
It is also important to confirm the size against the vehicle’s placard and owner’s manual. The U.S. National Highway Traffic Safety Administration (NHTSA) provides consumer tire-safety information, including guidance on tire pressure and maintenance. Tire manufacturers and qualified retailers can answer product-specific fitment questions.
Quick Answers About 315 Tires
- How wide is a 315 tire in inches? Its nominal section width is about 12.4 inches.
- Is a 315 tire 31.5 inches tall? No. The 315 number is width in millimeters, not height.
- How tall is a 315 tire? The answer depends on the aspect ratio and wheel diameter. A 315/70R17 calculates to about 34.4 inches overall.
- Does every 315 tire measure exactly 12.4 inches wide? No. Actual dimensions vary by product and measuring conditions.
- Can a 315 tire fit any 17-inch wheel? No. The tire must be compatible with the wheel’s width, and the full combination must fit the vehicle and meet its load requirements.
In short, “315 tire” means approximately 12.4 inches of nominal width. To determine sidewall height, total diameter, and vehicle fitment, the complete tire size and product specifications are essential.