Understanding the 315/70R17 Tire Code
The tire size 315/70R17 specifies critical dimensions for vehicle fitment and performance. Understanding this code is fundamental for selecting the correct rubber for your needs. This standard metric notation breaks down into three key components: tire width in millimeters, aspect ratio (sidewall height as a percentage of width), and rim diameter in inches.
- 315 is the tire width in millimeters.
- 70 is the aspect ratio (sidewall height is 70% of width).
- R17 indicates a radial tire for a 17-inch diameter wheel.
- This size generally translates to a large diameter tire.
When you see 315/70R17, the '315' is the first crucial piece of information. It represents the tire's width from sidewall to sidewall in millimeters. This measurement is taken at the widest point when the tire is mounted and inflated, but not under load. For a 315mm tire, this translates to approximately 12.4 inches.
Tire Width: The First Number
This width is significant for several reasons. A wider tire generally offers a larger contact patch with the road surface, which can improve traction, braking, and cornering stability, particularly on dry pavement. However, it can also increase rolling resistance, potentially affecting fuel economy. It also dictates the required rim width for proper mounting and can influence steering feel.
The sheer width of a 315mm tire means it is often found on heavy-duty trucks, SUVs, and performance vehicles designed to handle increased loads and provide aggressive styling. Such precision is paramount when considering suspension and fender clearance.
The 315mm width signifies a substantial tire, often chosen for enhanced grip and a commanding presence.
Aspect Ratio: The Second Number
Following the width is '70'. This number is the aspect ratio, expressed as a percentage. It indicates the tire's sidewall height from the rim flange to the tread, relative to the tire's width. So, for a 315/70R17 tire, the sidewall height is 70% of 315mm.
Calculating this gives us a sidewall height of 220.5mm (315mm * 0.70). This means the tire has a relatively tall sidewall compared to very low-profile tires. A taller sidewall offers a more compliant ride, absorbing road imperfections better, which is often desirable for off-road applications or for vehicles carrying heavy loads. It also provides more cushioning against impacts.
This mechanism is critical for ride comfort and off-road capability.
Rim Diameter: The Final Number
The 'R' in 315/70R17 signifies that the tire is of radial construction, which is standard for virtually all modern passenger vehicle tires. The '17' following the 'R' is the rim diameter in inches. This number tells you the size of the wheel the tire is designed to fit. A 17-inch tire must be mounted on a 17-inch wheel.
Choosing the correct rim diameter is non-negotiable; an incorrect size will prevent proper mounting and compromise safety. The combination of a 315mm width, a 70 aspect ratio, and a 17-inch rim diameter results in a tire with a substantial overall diameter, typically around 34.5 to 35 inches.
Ensure your vehicle's wheel wells and suspension components have sufficient clearance for the larger overall diameter of 315/70R17 tires before installation.
Calculating Overall Tire Diameter
You've learned that 315/70R17 means a width of 315mm, a sidewall height that's 70% of that width, and a 17-inch rim diameter. Now, let's translate these numbers into the tire's overall diameter, which is a key factor for fitment and speedometer accuracy.
The overall diameter is calculated by adding the height of the sidewall on both sides of the wheel to the rim diameter. We already calculated the sidewall height as 220.5mm. To convert this to inches, we divide by 25.4 (since 1 inch = 25.4mm):
Sidewall Height (inches) = 220.5 mm / 25.4 mm/inch ≈ 8.68 inches.
Since there are two sidewalls (top and bottom), the total sidewall contribution to the diameter is approximately 8.68 inches * 2 = 17.36 inches.
Now, add this to the rim diameter:
Overall Diameter ≈ 17.36 inches (sidewalls) + 17 inches (rim) ≈ 34.36 inches.
This calculation shows that a 315/70R17 tire typically measures around 34.5 inches in diameter. This figure is crucial for understanding how the tire will fit on your vehicle and its impact on gearing and speedometer readings. A common mistake is overlooking this total size.
The approximate 34.5-inch overall diameter is the most critical number for determining suspension and body clearance.
Understanding this principle is fundamental. If you change tire sizes, your speedometer will read inaccurately. For example, larger tires will make your vehicle travel further than the speedometer indicates, meaning you're actually going faster than you think. Conversely, smaller tires will make you go slower than indicated.
This difference in diameter, often referred to as tire growth or tire scrub, can also affect the performance of ABS and stability control systems, which rely on wheel speed sensors to function correctly. Our analysis indicates that significant deviations can trigger warning lights.
Calibrate your speedometer and odometer after installing tires with a significantly different overall diameter to ensure accuracy and compliance.
Practical Considerations and Fitment
What size tire is 315/70R17 usually fitted to, and what are the implications?
Tires of this size are most commonly found on full-size trucks and SUVs, particularly those equipped for off-roading or towing. Examples include models like the Ford F-150 Raptor, Jeep Wrangler Rubicon (often in slightly different sizes like 315/70R17), Toyota Tundra, and Chevrolet Silverado. They are favored for their ability to provide increased ground clearance, better traction in varied conditions (mud, sand, snow), and a more aggressive stance.
However, fitting 315/70R17 tires often requires modifications. The primary challenge is ensuring adequate clearance. The increased width can interfere with fenders, suspension components, and steering stops. The larger overall diameter can cause the tire to rub against the fender liner, especially during suspension compression or when turning the steering wheel.
Common Fitment Challenges
To accommodate tires of this size, many vehicle owners opt for suspension lifts or body lifts. A suspension lift raises the vehicle's body and chassis by modifying the suspension components, typically providing more room for larger tires. A body lift, on the other hand, inserts spacers between the frame and the body, lifting the body higher off the frame and creating clearance without altering suspension geometry.
In some cases, fender liner modifications or 'trimming' might be necessary. This involves cutting away parts of the plastic fender liner to prevent rubbing. Some aftermarket wheels with different offsets can also help tuck the tires in or push them out to achieve better clearance, though this must be done carefully to avoid impacting steering or suspension geometry.
It is imperative to acknowledge that simply bolting on 315/70R17 tires without considering these factors can lead to significant issues, including tire rubbing, compromised handling, and potential damage to vehicle components.
Proper fitment for 315/70R17 tires often involves suspension modifications to prevent rubbing.
When considering this tire size, think about the vehicle's intended use. For daily driving on paved roads, the increased tire noise, potential decrease in fuel efficiency, and altered handling characteristics might be drawbacks. For off-road enthusiasts or those who frequently tow heavy loads, the benefits of increased traction, ground clearance, and a more robust tire often outweigh these considerations. Always consult your vehicle's manual or a qualified tire professional for specific fitment advice.