How to read a semi truck tire size, and why it matters at the roadside

Two ways of writing a size

Commercial truck tire sizes are written in two systems, and both are common on the same highways. The older one describes the tire in inches: 11R22.5 or 11R24.5. The newer one uses metric width and a ratio: 295/75R22.5 or 285/75R24.5. Once you know how each is built, reading either takes seconds, and it is the single most useful thing a driver can tell a tire service before it rolls.

The letters and numbers are molded into the sidewall, usually in the largest text on the tire apart from the brand name. If the damaged tire's sidewall is torn or hard to reach safely, the matching tire on the same axle almost always carries the same size.

Reading an inch size: 11R22.5

An inch size reads left to right as width, construction and wheel diameter. The same pattern gives 11R24.5, a tire of similar width built for a taller 24.5-inch wheel. Taking 11R22.5 apart:

• 11 is the nominal section width of the tire, in inches: roughly how wide it is from sidewall to sidewall when mounted and inflated.
• R means radial construction, which is standard on modern heavy truck tires.
• 22.5 is the diameter, in inches, of the wheel the tire is built to fit. A 22.5 tire goes on a 22.5-inch wheel and nothing else, and the wheel diameter is the one part of a size that can never be mixed.

Reading a metric size: 295/75R22.5

A metric size puts a ratio in the middle, and it is not simply an inch size written in different units. Sizes such as 11R22.5 and 295/75R22.5 are close in overall dimensions and are often discussed together, but close is not identical: their overall diameter and load capacity can differ, which matters most when tires are paired on a dual assembly or run on the same axle. Taking 295/75R22.5 apart:

• 295 is the section width in millimeters.
• 75 is the aspect ratio: the height of the sidewall is 75 percent of the tire's width. A lower number means a shorter, lower-profile sidewall.
• R again means radial construction.
• 22.5 is the wheel diameter in inches, exactly as in the inch system.

Wide-base singles

Some trucks and trailers replace a pair of dual tires with a single, much wider tire, often called a wide-base single or super single. A common example is 445/50R22.5: 445 millimeters wide with a sidewall half as tall as that width, on a 22.5-inch wheel. Wide-base singles save weight and can save fuel, but they need their own wheels and they are a less common size to find at the roadside, so it is especially important to mention them when you call. At the roadside, a failed wide-base single is replaced with another tire of the same size; converting that position back to duals takes different wheels and hardware, which makes it a shop decision.

The numbers after the size

A size tells you what fits. The markings near it tell you what the tire is rated to carry, and on a commercial truck that matters as much as the fit.

• Load index. Commercial tires often show two load index numbers together, such as 146/143. The first is the rated load when the tire runs single; the second is the rated load per tire when it runs as part of a dual pair. Each number corresponds to a maximum load in a standard table.
• Speed symbol. A letter after the load index, such as L, indicates the speed the tire is rated for at its rated load.
• Load range or ply rating. Letters such as G or H, or a ply rating, describe the tire's strength and the inflation pressure it is designed for. A replacement should match or exceed what the position requires.
• DOT code. The DOT marking ends in a four-digit date code: the first two digits are the week and the last two the year the tire was made, so 2324 means the 23rd week of 2024.

Tread depth rules and tread types

Federal rules for commercial trucks set minimum tread depths. Tires on the front steer axle must have at least 4/32 of an inch of tread; other tires must have at least 2/32 of an inch. A tire worn below those depths is a violation, and a steer tire close to the limit is worth replacing before it becomes one.

Tread designs are also built for positions. Steer tires usually have ribbed treads designed for steering precision and even wear. Drive tires usually have deeper lug treads for traction. Trailer tires are designed for free-rolling positions and resistance to scrubbing. Some tires are designed as all-position. A tire designed for one position can wear quickly or handle poorly in another, so position is part of choosing a replacement, not just size.

Why duals need to match

On a dual assembly, two tires share the load of one wheel position. If one is noticeably larger in diameter, or carries more pressure, it takes more than its share of the load while its partner scrubs along the road and wears unevenly. Over time that overloads the bigger tire and wastes the smaller one. That is why a replacement on a dual position is matched to its partner in size, type and remaining tread as closely as practical, and why both tires are brought to the correct pressure afterwards.

It is also why a flat dual is more urgent than it looks. While one tire of a pair is flat, its partner is carrying the whole load on its own. Our post on how long a roadside tire replacement takes covers what happens once the service truck arrives.

What to read off the tire before you call

Our service truck carries commercial tires in the common sizes, so for most calls the right tire is already on board. When a truck runs an uncommon size or a wide-base single, telling us up front is what lets us source it before we set off. The mobile tire service page covers what we do at the roadside, and our post on what to do after a blowout covers the minutes before the call.

If you can read them safely, from the side of the truck away from traffic, note these before you call:

• The size, exactly as molded: 11R22.5, 295/75R22.5, 445/50R22.5 and so on.
• The position: steer, drive or trailer, which axle, which side, and inside or outside dual.
• The load index or load range, if you can read it safely.
• Whether it is a wide-base single.
• Any damage beyond the tire, such as a hot hub, broken studs, or brake lines hit by debris.

Related reading

How Long Does a Roadside Semi Tire Replacement Take? — The tire change is usually the shortest part of a roadside tire call.

Truck Tire Blowout on the Highway: Do These Five Things — Do not brake hard and do not fight the wheel with sudden corrections.

What Mobile Truck & Trailer Repair Costs in Kansas City — Mobile truck and trailer repair is priced per job rather than from a flat rate card, because the bill depends on the actual fault, the parts, how long diagnosis and repair take, and where the truck is sitting.