A 315 tire is about 12.4 inches wide, and its full diameter changes with the sidewall number and wheel size.
If you’re asking what a 315 tire size means in inches, the plain answer starts with width. The “315” on the sidewall means the tire’s section width is 315 millimeters, which converts to 12.40 inches. That gives you the width of the tire at its widest point, not the full height from ground to top.
That last part trips people up all the time. A tire marked 315/70R17 and a tire marked 315/35R20 are both 12.4 inches wide on paper, yet they stand at different heights once mounted. So when someone says “a 315 tire,” they’re usually talking about width, while the full size still needs the rest of the code.
What The 315 Number Really Means
Tire sizing follows a simple pattern once you know what each number does. In a size like 315/70R17, the first number is width in millimeters, the second is the sidewall ratio, and the last number is wheel diameter in inches. Goodyear’s tire size breakdown uses that same reading order.
Here’s the clean takeaway: 315 mm ÷ 25.4 = 12.40 inches. That tells you the tire is about 12.4 inches wide from sidewall to sidewall. It does not tell you tread width, and it does not tell you overall diameter.
Section Width Vs Tread Width
The 12.4-inch figure is section width. That’s measured across the widest part of the inflated tire, not the rubber patch touching the road. Tread width is often narrower, so a 315 tire will not always show a full 12.4 inches of tread.
Wheel width also changes the final mounted shape a bit. Put the same tire on a narrower rim and the sidewalls bulge more. Put it on a wider rim and the sidewalls flatten out. So 12.4 inches is the labeled width, not a promise that every mounted 315 will read the same with a tape measure.
315 Tire Size In Inches With Sidewall Math
Once you know the 315 number gives you width, the next job is finding sidewall height and full diameter. That takes only two small steps.
- Convert width: 315 mm ÷ 25.4 = 12.40 inches.
- Find sidewall height: width × aspect ratio.
Say the tire is 315/70R17. The sidewall height is 315 × 0.70 = 220.5 mm. Convert that to inches and you get about 8.68 inches. Since the tire has a sidewall above and below the wheel, you double that number and add the 17-inch wheel diameter.
How To Work Out Overall Diameter
The full formula is simple: overall diameter = wheel diameter + two sidewalls. For a 315/70R17, that means 17 + 8.68 + 8.68, which lands at about 34.36 inches. Same width, different aspect ratio, different height.
A 315/35R20 Worked Out
Now switch to 315/35R20. Width stays 12.40 inches. Sidewall drops to about 4.34 inches. Add the two sidewalls to a 20-inch wheel and the tire comes out to about 28.68 inches tall. That’s a huge change in height from the 315/70R17, even though both tires share the same 315 width label.
That’s why the answer to this topic has two layers. If you mean width, 315 equals 12.4 inches. If you mean the whole tire, you need the slash number and the wheel size too.
Common 315 Tire Sizes Converted To Inches
Below is a plain conversion chart for common 315 sizes. Each one has the same labeled width, yet the sidewall and full diameter shift a lot from one size to the next.
| Tire Size | Width / Sidewall (Inches) | Overall Diameter (Inches) |
|---|---|---|
| 315/30R20 | 12.4 / 3.72 | 27.44 |
| 315/35R20 | 12.4 / 4.34 | 28.68 |
| 315/40R21 | 12.4 / 4.96 | 30.92 |
| 315/45R22 | 12.4 / 5.58 | 33.16 |
| 315/60R18 | 12.4 / 7.44 | 32.88 |
| 315/70R17 | 12.4 / 8.68 | 34.36 |
| 315/75R16 | 12.4 / 9.30 | 34.60 |
| 315/80R22.5 | 12.4 / 9.92 | 42.34 |
A table like this shows why shorthand can be messy. Two drivers may both say they run “315s,” yet one setup sits low and wide while another sits tall and chunky. Width matches. Height may be nowhere close.
Why People Ask This Question In The First Place
Most readers are trying to solve one of three things: fitment, appearance, or speedometer change. They want to know whether a 315 tire will clear the fender, whether it will fit the wheel they already own, or whether it will throw off ride height and gearing.
Width alone won’t settle any of that. A 315 tire can be a short street tire, a tall off-road tire, or a heavy truck tire. The full code decides the rest.
- If you’re chasing stance, width is only one piece.
- If you’re chasing ground clearance, overall diameter matters more.
- If you’re chasing grip, tread design and compound still matter beside size.
There’s also the vehicle side of the story. Door placards and owner’s manuals list approved tire sizes for a reason. NHTSA’s tire advice on replacement sizes points drivers back to the placard, manual, or matching recommended size when buying new tires.
Checks To Make Before Buying A 315 Tire
A 315 sounds simple on paper. On a real vehicle, it can rub on control arms, liners, mud flaps, struts, sway bars, or the fender edge. Wheel offset and wheel width matter almost as much as the tire itself.
| Check Area | What To Measure | Why It Matters |
|---|---|---|
| Wheel Width | Approved rim range for the tire | A bad match changes tire shape and wear |
| Wheel Offset | How far the wheel sits in or out | Too much inboard or outboard room can vanish fast |
| Suspension Clearance | Gap to strut, arm, and sway bar | Inner rub shows up early on turns |
| Fender Room | Top and outer arch space | Compression can make the tire hit the body |
| Steering Sweep | Full-lock clearance left and right | Many rub issues show only at lock |
| Spare And Gearing | Overall diameter change | Ride height, speed reading, and gearing can shift |
If you’re swapping from a stock tire to a 315, the best move is to match the full diameter as closely as you can if you want the speedometer and gearing to stay near stock. If you want a taller setup, plan for the tradeoffs instead of finding them the hard way after the tires are mounted.
What To Say When Someone Asks About A 315 Tire
The cleanest answer is this: a 315 tire is about 12.4 inches wide. Then add one more sentence: the full height depends on the rest of the size code. That second line clears up the usual mix-up right away.
If you have the full size, you can work it out in under a minute. Width is fixed by the 315. The aspect ratio gives sidewall height. The wheel size finishes the diameter. Once you know that pattern, sidewall codes stop looking like alphabet soup and start reading like a simple formula.
So if someone asks, “What Is 315 Tire Size In Inches?” the honest answer is width first, then context. A 315 equals 12.4 inches across. Everything else hangs on the numbers that come after it.
References & Sources
- Goodyear.“How To Check Tire Size | Find Tire Size.”Shows how tire sidewall numbers are read, including width, aspect ratio, and wheel diameter.
- National Highway Traffic Safety Administration.“Tire Safety Ratings and Awareness.”States that replacement tires should match the size listed on the vehicle placard, owner’s manual, or approved fitment data.