Shopping for big truck or SUV tires often brings up the same question: what is the 315 tire size in inches? Tire shops list sizes in millimeters, while most drivers think in inches, and that mismatch causes real confusion.

This guide gives the quick answer first. Then it walks through the math, a full size chart, fitment rules, and common mistakes, so a buyer can choose with confidence.

What Does “315” Mean on a Tire?

The number 315 is the section width. It measures the tire from one sidewall to the other, in millimeters. It does not describe the height or the wheel size.

Here is how a full code such as 315/70R17 breaks down:

• 315: section width in millimeters (about 12.4 in).
• 70: aspect ratio. The sidewall height equals 70% of the width.
• R: radial construction, used on nearly all modern tires.
• 17: wheel diameter in inches.

Only the wheel diameter is already in inches. Everything else needs a conversion, which is where most searchers get stuck.

How to Convert a 315 Tire Size to Inches

The math takes four steps and works for any metric tire.

Step 1: Convert the Width

Divide 315 mm by 25.4 (millimeters per inch).

315 ÷ 25.4 = 12.4 inches wide.

Step 2: Find the Sidewall Height

Multiply the width by the aspect ratio. For a 315/70 tire:

315 × 0.70 = 220.5 mm, which is about 8.7 inches.

Step 3: Add the Wheel Diameter

The overall diameter equals the wheel size plus two sidewalls (top and bottom).

17 in + (2 × 8.68 in) = about 34.4 inches.

Step 4: Check the Circumference

Multiply the diameter by π (3.1416). A 34.4-inch tire has a circumference near 108 inches, or about 587 revolutions per mile.

315 Tire Size Chart in Inches

The table below lists common 315-width sizes. All values are calculated from the standard formula and rounded to one decimal place.

The width stays at 12.4 inches in every row. Only the aspect ratio and wheel size change the height. That is why two “315” tires can look completely different on the same truck.

315/70R17: The Most Popular 315 Tire Size

Among 315 tires, the 315/70R17 gets the most searches, mainly from owners of lifted pickups and off-road SUVs. Truck forums and fitment calculators such as the Tacoma World tire calculator discuss it as the metric twin of the 35×12.50R17.

315/70R17 Specifications

Typical Load and Construction

Many 315/70R17 tires are Load Range E truck tires. A common load index is 121, which supports roughly 3,197 lb per tire at maximum pressure. Exact ratings differ by model, so read the sidewall of the tire you plan to buy.

Wheel Width Guidance

Tire makers publish an approved rim range for each model. A 315 tire generally works best on wider wheels, often in the 9 to 10 inch range. Check the specific manufacturer’s data sheet, such as the Pirelli catalog page for this size, before mounting.

Best Vehicles for 315/70R17

• Lifted Toyota Tacoma, 4Runner, and Tundra builds
• Jeep Wrangler and Gladiator with a lift and wheel offset changes
• Ford Bronco with a suspension upgrade
• Full-size pickups such as the F-150, Silverado 1500, and Ram 1500

Most of these need a lift, trimming, or regearing to run a tire this tall without rubbing.

315 vs 285 vs 295 Tires: Comparison

Drivers often debate whether to step up to a 315 or stay with a narrower tire. Here is a side-by-side look at 17-inch options with a 70 aspect ratio.

Pros of a 315 Tire

• Larger contact patch for grip on sand, mud, and loose rock
• Extra ground clearance from the taller diameter
• A bold, aggressive stance

Cons of a 315 Tire

• More weight, which can reduce fuel economy and acceleration
• Higher chance of rubbing without a lift
• Higher purchase price than narrower sizes
• May require regearing and speedometer recalibration

Budget matters here, since big tires cost more. Readers can compare prices in our guide on how much a new tire costs.

Will a 315 Tire Fit Your Vehicle?

Fit depends on more than the size code. These factors decide whether a 315 tire clears the fenders and suspension.

1. Lift Height

A tire close to 35 inches tall usually pairs with a suspension lift. Many owners of mid-size trucks add a lift of about 3 inches or more, though requirements vary by vehicle. Confirm with a vehicle-specific forum or a trusted installer.

2. Wheel Offset and Backspacing

Offset moves the tire in or out. A wide 315 on a high-offset wheel can scrape the control arms. A lower offset can push the tire into the fender.

3. Fender Trimming and Bumper Clearance

At full steering lock and full suspension compression, the tire should clear every part of the body. Test this before final installation.

4. Brake and Axle Strength

Heavy tires add rotational mass and stress steering parts, ball joints, and axles. Plan to inspect these parts more often.

How a 315 Tire Changes Your Speedometer and Gearing

Larger tires cover more ground per rotation. The speedometer, however, still counts rotations based on the stock size.

Speedometer Error Example

Suppose a truck came with 265/70R17 tires (about 31.6 in tall). After switching to 315/70R17 (about 34.4 in tall), the tire is roughly 8.7% larger. The speedometer then reads low. When it shows 60 mph, the real speed is closer to 65 mph.

Gearing and Performance

A taller tire acts like a taller gear ratio. Expect softer acceleration, more strain while towing, and a lower mpg figure. Trucks with stock 3.73 gears often need regearing to restore performance.

Practical Tips for Buying 315 Tires

1. Confirm the real diameter. Brands differ by a few tenths of an inch. Look at the published spec, not just the nickname.
2. Match the load rating to the job. Heavy trucks and trailers need adequate load index and load range.
3. Pick the tread for your terrain. All-terrain tires suit mixed driving. Mud-terrain tires suit trails but are louder on pavement. For a well-known all-terrain option, see our review of the Nitto NT555 RII.
4. Check the tire’s age. Even new tires can sit in storage for years. Learn how in our guides on how to tell tire age and the DOT tire date code.
5. Read the warranty. Compare tread-life and road-hazard coverage. Our pages on the Michelin tire warranty and the Costco tire warranty are good starting points.
6. Budget for installation. Big tires cost more to mount and balance. See typical charges in Discount Tire mount and balance cost.
7. Do not forget the spare. A 315 spare is large and heavy. Learn about costs in how much a spare tire is.
8. Plan sensor updates. New wheels may need TPMS service. Read about the TPMS replacement cost.

Maintenance After Installation

• Check pressure monthly and use the vehicle placard as a starting point, adjusting for load per the tire maker’s guidance.
• Rotate regularly. See the Valvoline tire rotation cost for typical pricing.
• Ask about free repairs. Some shops handle simple punctures at no charge, as covered in does Discount Tire patch tires for free.

For official safety guidance on tire labels and maintenance, the NHTSA tire safety page is a reliable reference. Size standards are published by groups like the Tire and Rim Association.

Common Mistakes When Reading Tire Sizes

• Treating 315 as inches. It is millimeters. Dividing by 25.4 gives the width in inches.
• Assuming equal width means equal height. A 315/55R20 and a 315/70R17 share a width but differ by almost an inch in diameter.
• Ignoring the wheel. The tire must match the wheel diameter and an approved rim width.
• Skipping the speedometer fix. Larger tires change the reading.
• Buying by nickname only. “35-inch” tires can measure anywhere near 34 to 35.5 inches.

Conclusion

The 315 tire size in inches comes down to one simple conversion: 315 mm equals about 12.4 inches of width. Height is where it gets interesting. A 315/70R17 stands near 34.4 inches, close to the popular 35-inch class, while other 315 sizes range from about 29 to 35 inches.

Before buying, confirm the manufacturer’s real diameter, check wheel width and offset, plan for lift and gearing, and recalibrate the speedometer. With those steps, a 315 tire can deliver strong grip, extra clearance, and an aggressive look without surprises.