How to Read Tire Size: What 265/70R17 Actually Means

Here is how to read tire size in one breath: in 265/70R17, the 265 is the section width in millimetres, the 70 is the aspect ratio (sidewall height as a percent of that width), the R means radial construction, and the 17 is the wheel diameter in inches. That is the whole code. Everything else on the sidewall is extra detail stacked around those four numbers.

The reason this code feels harder than it is: it mixes metric and imperial units in a single string. Millimetres, then a percentage, then inches. Nobody would design it that way on purpose today, but it is what is molded into your tire, so it is worth ten minutes of your time.

What do the numbers on a tire mean?

Each part of the code answers a different question, and only one of them is a real measurement in the unit you would expect. Using 265/70R17 as the example:

• 265 = section width in millimetres. This is the tire measured across at its widest point, sidewall bulge to sidewall bulge. 265 mm works out to 10.43 inches.
• 70 = aspect ratio, a percent, not a measurement. The sidewall height is 70% of the section width.
• R = radial construction.
• 17 = wheel (rim) diameter in inches. This is the wheel the tire mounts on, not the tire itself.

Notice what is missing. Nowhere in 265/70R17 does the tire tell you how tall it is overall. That number, the one that actually matters for fitment and for your speedometer, has to be calculated. Which brings us to the math.

If you want the deep-dive on this one size specifically, we broke it down further in what 265/70R17 means.

What does the R stand for in a tire size?

The R stands for radial construction. That refers to how the internal plies, the cords under the rubber, are laid out. On a radial tire they run across the tire from bead to bead, roughly perpendicular to the direction of travel, with belts layered on top under the tread.

Radial is what nearly every passenger car, truck and SUV rolls on. You will occasionally see a D in that slot on older or specialty tires, which means bias-ply construction. If you are shopping for a normal vehicle at a normal shop, you will see R.

The R is the least interesting letter on your sidewall. It almost never changes, and it almost never causes a problem. Move on to the numbers.

Why does tire size mix millimetres and inches?

Because the code grew in pieces. The width came from metric engineering conventions. The rim diameter came from wheels that were already made and sold in inches, and still are. The aspect ratio sits in the middle as a percentage, which belongs to no unit system at all.

The practical effect: you cannot eyeball two tire codes and know which is taller. The percentage is the trap. Here is the key insight most people miss:

The aspect ratio is a percentage of the width, not a fixed height. So the same 70 means a different sidewall on a 265 tire than on a 285 tire. A 265/70 has a 185.5 mm sidewall. A 285/70 has a taller one, because 70% of a bigger number is a bigger number. Two tires with the same middle number are not the same height.

That is exactly why the numbers alone are not comparable, and why the Tire Size Calculator exists. You feed it section width in mm, aspect ratio and rim diameter, and it gives you overall diameter, circumference and revolutions per mile. More on aspect ratio in our piece on what tire aspect ratio really measures.

How do you calculate tire height and overall diameter?

Two formulas do all the work here, and neither is hard.

• Sidewall height (mm) = section width x aspect ratio / 100
• Overall diameter (in) = rim diameter + (2 x sidewall height converted to inches)
• Conversion: 25.4 mm = 1 inch

You double the sidewall because a tire has one above the rim and one below it. Now walk it through with 265/70R17:

• Sidewall: 265 x 0.70 = 185.5 mm
• Convert: 185.5 / 25.4 = 7.30 in
• Diameter: 17 + (2 x 7.30) = 31.61 in
• Width: 265 / 25.4 = 10.43 in
• Revolutions per mile: 638

So a tire whose code contains a 17 is actually about 31.61 inches tall. That gap between the number on the sidewall and the real height is where most tire-shopping mistakes start. People swap sizes assuming the rim number is the size. It is not. It is one input out of three.

Revolutions per mile is the number worth noting. It is how many times the tire turns to cover a mile, and your speedometer is calibrated around the original figure. Change the diameter and that count changes, which is a real consequence, not a trivia point.

How do common tire sizes compare?

Here is what the math produces across six sizes you will actually run into. Diameter, width, sidewall and revolutions per mile:

• 265/70R17 = 31.61 in diameter / 10.43 in wide / 7.30 in sidewall / 638 revs per mile
• 285/70R17 = 32.71 in / 11.22 in / 7.85 in / 617
• 265/65R17 = 30.56 in / 10.43 in / 6.78 in / 660
• 275/70R18 = 33.16 in / 10.83 in / 7.58 in / 608
• 225/45R17 = 24.97 in / 8.86 in / 3.99 in / 808
• 235/75R15 = 28.88 in / 9.25 in / 6.94 in / 698

Read that list slowly and a few things jump out. All three 17-inch entries have wildly different heights: 31.61, 32.71, 30.56 and 24.97 inches. Same rim number, four different tires. The 225/45R17 is nearly seven inches shorter than the 285/70R17 despite sharing that 17.

Compare 265/70R17 and 265/65R17. Same width, same rim, one number changed from 70 to 65, and the tire lost about an inch of height and gained 22 revolutions per mile. Small digit, real change. We ran the direct comparison in 265/70R17 vs 285/70R17.

And look at 235/75R15 versus 265/65R17. The 15-inch tire has a taller sidewall (6.94 in vs 6.78 in) than the 17-inch one. The rim number told you nothing about the rubber.

What does a size like 33x12.50R17 mean?

That is a flotation size, and it is the honest one. Flotation sizing skips the percentage games and states the dimensions directly in inches:

• 33 = overall diameter in inches
• 12.50 = width in inches
• 17 = rim diameter in inches

No conversion, no math, no aspect ratio. A 33x12.50R17 is a 33-inch-tall tire on a 17-inch wheel. You mostly see this format on light truck and off-road tires, and it is why people who lift trucks talk in inches while everyone else talks in three-digit codes.

This is also the bridge between the two systems. If someone asks whether your 265/70R17 counts as a 33, the answer is no, it computes to 31.61 inches. Close-ish, not the same. See what size tire is 33 inches for the metric equivalents.

What are the load index and speed rating?

Those are the characters that follow the size, like the 116T in 265/70R17 116T. They are two separate things stuck together.

The load index is a code number, not a weight. It maps to a maximum weight capacity through a standard chart. The number itself is not pounds or kilos, so 116 does not mean 116 of anything.

The speed rating is the letter. It maps to a maximum sustained speed, again through a standard chart.

We are not going to print those charts here, and you should be suspicious of any page that reproduces them from memory. Get the values from the tire manufacturer chart, and check the placard in your driver door jamb for what your vehicle requires. Guessing on load capacity is not a place to be creative, especially if you tow or haul.

How do you find your tire size?

Two places, and you should check both.

The tire sidewall. Walk out and look. The code is molded into the rubber in raised letters, usually the largest text on the tire. This tells you what is currently fitted, which is not always what should be fitted, especially on a used vehicle.

The driver door jamb placard. Open the driver door and look at the frame. There is a sticker with the vehicle manufacturer's recommended sizes and inflation pressures. This is the authority. If the sidewall and the placard disagree, someone changed the tires and the placard is what your vehicle was engineered around.

Owner's manual and glovebox door are the backups. But the placard is the one that settles arguments.

Why does overall diameter matter more than the numbers suggest?

Because diameter drives three things you feel and one thing you get ticketed for.

Speedometer accuracy. Your speedometer counts wheel rotations and assumes a fixed distance per rotation. Fit a taller tire, cover more ground per turn, and the readout runs low. A 265/70R17 turns 638 times per mile; a 285/70R17 turns 617. Same road, different count. Details in how tire size affects your speedometer.

Clearance. Taller and wider tires need somewhere to go when you turn and when the suspension compresses. Some sizes will not fit without rubbing on the fender liner, the control arm or the frame. Some need a lift to clear at all.

Handling. Sidewall height changes how the tire flexes. A 3.99-inch sidewall (225/45R17) responds very differently from a 7.85-inch one (285/70R17).

The rule: stay within the vehicle manufacturer's recommended sizes unless a tire professional confirms the change for your specific vehicle. Check the door placard first. Going wider on your existing wheels has its own limits, which we cover in putting a wider tire on the same rim.

Before you commit to anything, run both sizes through the Tire Size Calculator and compare the diameters and revolutions per mile side by side. Numbers first, then the shop.

Bottom line

Reading tire size comes down to four slots: width in millimetres, aspect ratio as a percent, R for radial, rim diameter in inches. The percentage is the part that fools people, because 70% of 265 and 70% of 285 are not the same sidewall. Do the math or let a tool do it. Confirm your size on the door placard, get load index and speed rating from the manufacturer chart, and treat any size change as a fitment question rather than a shopping preference.

Plug your numbers into the Tire Size Calculator to get overall diameter, circumference and revolutions per mile in a couple of seconds.