What Size Tire Is 315 70r17

Did you know that switching to a 315/70R17 tire can add nearly two inches of ground clearance to your truck without requiring a massive suspension lift? Most owners assume bigger is always better, but understanding the precise math behind these numbers is what separates a capable rig from one that constantly rubs against the wheel well. This tire size isn’t just a random label; it’s a specific engineering specification that defines how your vehicle handles off-road obstacles and highway fuel efficiency.

What are the exact physical dimensions of a 315/70R17 tire?

A 315/70R17 tire measures approximately 34.4 inches in total diameter and 12.4 inches in width. The “315” represents the section width in millimeters, while the “70” is the aspect ratio, meaning the sidewall height is 70% of that width. When you multiply 315 by 0.70, you get a sidewall height of 220.5 millimeters, or about 8.68 inches. Adding two of these sidewalls—top and bottom—plus the 17-inch rim diameter gives you that calculated height of roughly 34.4 inches. Actually, let me rephrase that—depending on the specific tire manufacturer, the unladen diameter often fluctuates between 34.3 and 34.6 inches due to tread depth variations.

Why do off-road enthusiasts choose this specific size?

Drivers choose 315/70R17 tires because they provide the ideal balance between aggressive trail capability and daily driver comfort. This size is famously known in the off-roading community as the “metric 35.” While a true 35-inch tire often requires extensive trimming or heavy suspension modifications, the 315/70R17 usually clears stock or mildly leveled fenders with minimal fuss. Unexpectedly: many people notice that this size offers better flotation on sand because the 12.4-inch width distributes the truck’s weight over a larger footprint compared to narrower 33-inch tires.

How does the 315/70R17 affect your speedometer and odometer?

Changing to this larger tire will cause your speedometer to read lower than your actual speed, typically by about 5% to 7% if you were previously running stock 32-inch tires. I remember upgrading my own vehicle and being surprised when a radar sign flashed 72 mph while my gauge showed only 68 mph. This discrepancy occurs because the tire has a larger circumference, meaning it covers more ground with every single rotation. Without recalibrating your truck’s computer using an OBD-II programmer, your odometer will also log fewer miles than you are actually traveling.

What is the impact on your vehicle’s fuel economy?

Expect a noticeable drop in gas mileage, typically ranging from one to three miles per gallon. The increased mass of these tires requires more energy to get moving from a stop, adding rotational inertia that your engine must fight against. A colleague once pointed out that the aggressive, open-lug tread pattern found on most 315/70R17 off-road tires creates significant wind resistance at highway speeds, further punishing your fuel efficiency. If you are doing mostly city driving, the stop-and-go nature of the commute makes this loss even more pronounced.

Do you need a lift kit to run these tires?

You often don’t need a full suspension lift, but a leveling kit is almost always required to prevent rubbing during full-lock turns. Most modern half-ton trucks like the Ford F-150 or Ram 1500 have enough room in the wheel wells to accommodate the diameter, but the width is the sneaky culprit. When I tested this setup on a stock truck, the tires grazed the sway bar during sharp turns, creating a metallic grinding sound that was impossible to ignore. A slight spacer or a wheel with a more aggressive offset—typically around +10mm or 0mm—will move the rubber away from the internal suspension components.

Are there specific wheel requirements for this tire size?

Your wheels must be 17 inches in diameter, but the rim width is the secret factor that determines performance. A rim width between 8 and 10 inches is standard for this tire. If you install a 315/70R17 on a narrow factory wheel, the sidewalls will bulge, leading to uneven tread wear and a “mushy” feeling in corners. I’ve seen this firsthand where a driver used an 7.5-inch wide rim and couldn’t get the center of the tread to touch the road properly even at lower pressures. Stick to the manufacturer’s recommended rim width range to ensure the contact patch remains flat.

What most overlook regarding tire pressure?

Most drivers treat tire pressure as a “set it and forget it” task, yet this size requires careful adjustment to avoid uneven wear. Because these tires have a higher load capacity, running them at the maximum PSI listed on the sidewall—often 65 or 80 PSI—will make your truck feel like it’s riding on solid granite slabs. Instead, perform a chalk test. Draw a line across the tread, drive a short distance, and observe where the chalk wears off. If the center wears away faster, you are over-inflated; if the edges vanish, your pressure is too low. Finding that sweet spot is the only way to get 50,000 miles out of a set.

How does the weight of these tires affect braking?

Adding these heavy tires increases your vehicle’s unsprung weight, which directly impacts your stopping distance. A standard 315/70R17 all-terrain tire often weighs between 60 and 75 pounds, which is significantly heavier than a stock tire. You are essentially increasing the amount of mass that your brake system must bring to a halt. When driving in heavy traffic, you should consciously increase your following distance by at least two car lengths to compensate for the extra momentum. This is a simple change that keeps you and your passengers safe.

Is the 315/70R17 the right choice for towing?

Towing with this size is entirely possible, but you will feel a loss of power because of the effective change in your final drive ratio. The larger tires act like a taller gear, making it harder for the engine to pull heavy loads up steep grades. If you tow frequently, you might eventually need to re-gear your differentials to bring the performance back to factory levels. Still, the extra stability provided by the wider stance is a benefit many towers appreciate once they get the rig rolling down the highway.

Why should you verify your tire load index?

Always check the load index, such as 113 or 121, to ensure the tire is rated for your truck’s Gross Vehicle Weight Rating. A tire with a “C” load rating will ride smoother than an “E” rated tire, but it might flex too much under a heavy payload. If you haul a slide-in camper or heavy utility trailer, opting for the higher ply count is not optional; it is mandatory for stability. Don’t sacrifice safety for a slightly softer ride on the street.

Ultimately, choosing the 315/70R17 is a commitment to a specific style of driving that prioritizes grit over pure efficiency. If you aren’t willing to sacrifice a little bit of fuel and adjust your driving habits, you might be better off sticking with stock rubber. But if you value the look and the undeniable advantage on uneven terrain, there is simply no substitute for this classic size.