What Are 315 Tires In Inches? | Width And Height Math

A 315 tire is about 12.4 inches wide, while total height changes with the sidewall ratio and wheel diameter.

If you’re asking what are 315 tires in inches, start with the first number only. In metric tire sizing, 315 means the tire’s section width is 315 millimeters. Divide 315 by 25.4, and you get 12.40 inches. That gives you the tire’s paper width, not its full height.

The part that trips people up is the rest of the size. A tire marked 315/70R17 is still 12.4 inches wide on paper, but it is much taller than a 315/35R20. So when someone says “a 315 tire,” the true answer is never just one inch number. Width is fixed. Height depends on the rest of the code.

What Are 315 Tires In Inches? Width, Sidewall, And Diameter

Here’s the clean way to read it:

• 315 = section width in millimeters
• 70 = sidewall height as a percent of width
• R17 = a 17-inch wheel

That means 315 converts to 12.40 inches wide. Then the sidewall height comes from 70% of 315 mm. After that, you add the top sidewall, the bottom sidewall, and the 17-inch wheel to get total tire height.

How The Math Works

You can do the conversion in three short steps:

1. Convert width to inches: 315 ÷ 25.4 = 12.40 inches
2. Find one sidewall: 315 × 0.70 = 220.5 mm, or 8.68 inches
3. Find total height: 8.68 + 8.68 + 17 = 34.36 inches

So a 315/70R17 is about 34.4 inches tall. That’s why many truck owners treat it like a near-35-inch tire.

One Worked Example

Take a 315/75R16. The width is still 12.40 inches. One sidewall is 315 × 0.75, which equals 236.25 mm, or 9.30 inches. Double that sidewall and add the 16-inch wheel, and the full tire lands at 34.60 inches tall.

A lot of searchers expect the first number to act like flotation sizing, where a tire might be labeled 35×12.50R17. Metric sizing does not work that way. In a metric size, 315 is width only. It does not mean 31.5 inches tall, and it does not tell you the full diameter by itself.

That’s why a 315 can show up on wildly different vehicles. On a lifted truck, 315/70R17 or 315/75R16 usually means a tall, meaty tire with a large sidewall. On a street car, 315/35R20 means a wide tire with a much shorter sidewall. Same width. Different stance. Different clearance needs. Different speedometer effect.

If you want the official breakdown of what each size number means, Goodyear’s tire size explanation lays out the width, aspect ratio, and rim diameter in the same order you see on the sidewall.

Common 315 Tire Sizes In Inches

All of the sizes below share the same paper width: 12.40 inches. What changes is the sidewall and the full diameter. This is where a lot of fitment mistakes start. Two tires can both say 315 and still sit in much different ways under the fender.

There’s a clean shortcut here. If the second number rises, the tire gets taller. If the wheel number rises while the sidewall ratio stays the same, total diameter grows too. That is why 315/75R17 towers over 315/35R20, even though both start with the same 315 width. Read the whole code, and the inch conversion stops being a guessing game.

Tire Size	Sidewall Height	Overall Diameter
315/30R22	3.72 in	29.44 in
315/35R20	4.34 in	28.68 in
315/35R21	4.34 in	29.68 in
315/40R21	4.96 in	30.92 in
315/60R20	7.44 in	34.88 in
315/70R17	8.68 in	34.36 in
315/75R16	9.30 in	34.60 in
315/75R17	9.30 in	35.60 in

That table tells the whole story. “315” gives you width. The second number decides whether the tire is low and wide, tall and chunky, or somewhere in the middle.

Why A 315 Tire May Not Measure Exactly 12.4 Inches

This is where real-world tire sizing gets messy. The 12.40-inch figure is the branded section width. It is not a promise that every mounted 315 will measure that number with a tape measure on your truck.

Mounted size changes with wheel width, air pressure, load, and the tire maker’s casing shape. Tire Rack’s tire-dimension formula shows the basic diameter math, and its spec notes also point out that mounted width is tied to a measuring rim. Go one wheel size wider or narrower than that test rim, and the tire’s real section width can shift a bit.

Section Width And Tread Width Are Not The Same

Another easy mix-up: section width is the widest point of the inflated tire, measured from sidewall to sidewall. Tread width is the rubber that actually sits on the road. A 315 tire can have a tread width that is well under 12.4 inches. That is normal.

So if you’re checking fender clearance, upper control arm clearance, or rubbing on frame horns, don’t shop by the 315 number alone. Look at the maker’s published specs for section width, measured rim width, and overall diameter.

315 Tires In Inches On Trucks, SUVs, And Street Setups

On trucks and off-road builds, a 315 width usually signals a wide, aggressive tire. The most common question there is not width. It’s whether that 315 works out to a 34-inch tire, a 35-inch tire, or something close enough to either one.

Here’s the fast read on the sizes people ask about most:

• 315/70R17 = about 34.4 inches tall
• 315/75R16 = about 34.6 inches tall
• 315/75R17 = about 35.6 inches tall
• 315/60R20 = about 34.9 inches tall

On performance cars, the story changes. A 315/30R22 and a 315/35R20 are still wide tires, but they carry much shorter sidewalls. That gives a flatter, lower look and a shorter overall diameter than the truck sizes above.

Metric Size	Think Of It As	What That Means
315/70R17	Near a 35×12.50R17	Wide and tall, but a hair under 35
315/75R16	Near a 35×12.50R16	Old-school truck fitment with strong sidewall height
315/75R17	Past a 35×12.50R17	Taller than many “35” labels suggest
315/60R20	Near a 35×12.50R20	Large wheel, still close to 35-inch height

Fitment Checks Before You Buy

Width and height are only part of the call. Before you order 315s, check these points:

• Wheel width: A 315 usually needs a wheel wide enough to seat the tire correctly.
• Clearance: Look at the frame, control arms, sway bar, liner, mud flap, and fender lip.
• Load rating: Two 315 tires can carry different loads.
• Speedometer change: A taller tire travels farther per turn, so the speed reading can drift.
• Spare fit: A 35-inch-class tire may not fit where the stock spare sat.

That last point gets missed all the time. A set of 315s may bolt on, but the spare, the jack setup, and the tailgate swing can still turn into a headache.

Wheel width deserves a close look too. A tire that is too pinched can crown in the center. A tire stretched too far can change the sidewall shape and the way the tire sits in the wheel well. Even when two builds run the same 315 size, the mounted look can change a lot if one truck is on an 8.5-inch wheel and the other is on a 10-inch wheel.

Then there’s diameter creep. Some brands run a touch taller or wider than the math says once mounted and inflated. That does not mean the size code is wrong. It means tire specs are built around test conditions, and real fitment still needs a tape measure and a careful check around lock-to-lock steering travel.

The Clearest Way To Read A 315 Tire

If all you wanted was the inch conversion, here it is: 315 mm equals 12.40 inches. That is the width. The full tire height comes from the rest of the size code, so a 315 tire can land anywhere from the high-20-inch range to the mid-35-inch range.

That’s why the smart way to shop is to ask for the full size, not just “315s.” Once you know the aspect ratio and wheel diameter, you can tell whether the tire will sit low and wide, stand tall like a near-35, or push past that mark.