How to Read a Tire Size

Two systems, because one would have been too easy

There are two ways to write a truck tire size, and they don't agree on units, order, or common sense. Metric (265/70R17) packs millimeters, a percentage, and inches into one string. Flotation (35x12.50R17) just tells you the dimensions in inches. Here's how to read both, and how to turn one into the other without a slide rule.

The metric format (265/70R17)

Most trucks and SUVs leave the factory on metric sizes. Read 265/70R17 left to right.

265 is the section width in millimeters, sidewall to sidewall. Millimeters, on a tire that bolts to a wheel measured in inches. Hold that thought.

70 is the aspect ratio: the sidewall height as a percentage of the width. Not a measurement, a percentage of a different measurement. So the 70 tells you nothing until you multiply it by the 265. A lower number means a shorter sidewall.

R means radial construction. That's nearly every street and all-terrain tire, but not every mud tire. More on that in a second.

17 is the wheel diameter, in inches. Because why would the last number use the same units as the first one.

A letter in front tells you the class. P is passenger-rated. LT is light truck, built with stronger construction for heavier loads and towing.

Turning metric into inches (the part nobody prints on the sidewall)

Multiply the width by the aspect ratio: 265 × 0.70 = 185.5 mm of sidewall, or 7.3 inches. A tire has two sidewalls, top and bottom, so that's 14.6 inches. Add the 17-inch wheel and you've got a 31.6-inch tire. That's the number you actually wanted, and it's the one number the size doesn't tell you.

The truck tire size chart has the math done for every common truck size, and the tire size calculator will do it for any two sizes you throw at it, speedometer error included.

The flotation format (35x12.50R17)

Flotation sizing is the one that respects your time. 35 is the overall height in inches, 12.50 is the width in inches, R is radial, and 17 is the wheel. That's it. No multiplying, no percentages, no metric conversion.

It's why nobody at the trailhead says "I'm running 315/70s." They say "35s," because height is the number that matters. For the record, a 315/70R17 is 34.4 inches tall. Close enough for bragging rights, not close enough to promise fender clearance.

No R? You might be looking at a bias-ply

Plenty of mud and competition tires are bias-ply, and they skip the R. You'll see a dash or an x instead: 35x12.50-15 or 35x12.50x17. Same idea, different construction, and in deep mud that construction is often the point. Our guide to bias-ply vs radial covers when bias beats radial.

Fair warning: bias sizing gets creative. Some Super Swampers list width first, so a 16x35x15 is 35 inches tall and 16 wide, not the other way around. And a few bog tires still wear 1970s letter sizes like N78x15 or Q78x15, which decode into nothing without a chart. NTW's listings usually print the inch size right next to it, for which we're all grateful.

Why is it called "flotation"?

Not because the tires float. Well, sort of. The format came from wide, low-pressure off-road and light-truck tires built to spread a truck's weight over a bigger footprint, so it stays on top of sand, mud, and snow instead of digging in. Same idea as snowshoes. Those tires were sized in plain inches, the name stuck to the format, and now a 35x12.50R20 on a pickup that never leaves pavement is technically a flotation tire. It just floats over the Costco parking lot.

Load index and speed rating

After the size you'll see something like 116T. The 116 is a load index, a code you look up in a table to get the maximum weight per tire, because a third unit system was apparently needed. A 116 means 2,756 pounds. The T is the speed rating.

If your truck tows or hauls, don't ignore the load index. Running one that's too low is a safety problem, and it's the one part of the sidewall that isn't funny.

Confirming fitment

Taller isn't automatically better. Before upsizing, check that the new tire clears your suspension and fenders at full lock and full articulation. Remember that a taller tire makes your speedometer read low and softens your effective gearing; the calculator shows exactly how much. When in doubt, confirm against your lift or leveling kit before you buy.