How to Read a Tire Size, Load Index and Speed Rating

The size molded on a tire’s sidewall packs width, sidewall height, construction, wheel size, load capacity and speed rating into one short code. Misread one part and you can end up with a tire that doesn’t fit the wheel, rubs the body or can’t carry the vehicle’s weight. Here is how to read the four size formats you are most likely to see.

Find your size on the placard, not just the old tire

Your vehicle maker lists the original tire size and the recommended cold inflation pressure on the Tire and Loading Information placard. On most vehicles it is on the driver’s door edge or door jamb; some put it on the glove-box door or inside the trunk lid. The owner’s manual has the same information.

Reading the tires already on the vehicle is a useful cross-check, but don’t rely on it alone. A previous owner or shop may have fitted a different size, and some vehicles use different sizes front and rear. If the placard and the tires disagree, go with the placard or ask a tire professional why they differ.

Decoding a passenger size: P225/65R17 100H

Sizes that start with P follow the Tire and Rim Association (TRA) standard used in North America. Read them left to right:

• P: passenger car tire.
• 225: section width in millimeters, measured from the outside of one sidewall to the outside of the other.
• 65: aspect ratio. Sidewall height is 65% of the section width, about 146 mm or 5.8 inches here. A lower number means a shorter sidewall.
• R: radial construction.
• 17: rim diameter in inches. The tire fits only a wheel of that diameter.
• 100: load index. Index 100 stands for 1,764 lb per tire, and in the TRA tables this size reaches that load at 35 psi.
• H: speed symbol, which stands for 130 mph.

Together, the load index and speed symbol are called the service description. The size also tells you roughly how tall the tire is: two sidewalls of about 5.8 inches plus the 17-inch wheel gives an overall diameter near 28.5 inches.

Euro-metric and XL sizes: same layout, different tables

A size with no letter in front, such as 225/55R17 97T, is a metric size built to the European Tyre and Rim Technical Organisation (ETRTO) standard. It reads the same way, but its load tables differ. One tire maker’s load-table guide gives an example: a P225/55R17 95T carries up to 1,521 lb at 35 psi, while a 225/55R17 97T carries up to 1,609 lb at 36 psi. The same guide warns that an equal or higher load index does not always mean equal or higher capacity at every pressure, especially between P-metric and Euro-metric tires.

You may also see XL, Extra Load or Reinforced on the sidewall. These tires carry more than a standard-load tire of the same size, but only at higher pressure. In the industry tables, standard-load passenger tires reach their maximum load at 35 psi (P-metric) or 36 psi (Euro-metric), while extra load tires keep gaining capacity up to 41 or 42 psi. A tire with none of those markings is standard load.

Light truck sizes: LT275/70R18 125/122S

An LT prefix marks a light truck tire built to the TRA light truck standard. Width, aspect ratio, construction and rim size read just like a passenger size. The differences are in the load markings:

• 125/122: two load indexes. The first applies when one tire sits at each end of an axle. The second applies when tires are mounted in pairs, as on dual-rear-wheel trucks. In the TRA light truck table, this size carries 3,640 lb as a single or 3,305 lb per tire in duals, both at 80 psi.
• S: speed symbol, 112 mph.
• Load range: a letter such as C, D, E or F, molded on the sidewall and often listed right after the size, as in LT275/70R18 E. A higher letter allows higher pressure and more load. This size reaches its 125/122 rating in load range E.

LT tires also need their own pressure. Size for size, an LT tire needs more air than a P-metric tire to carry the same weight, so the pressure you ran on passenger tires does not carry over. A tire shop can look up the right figure in the load tables.

Flotation sizes for lifted trucks: 35×12.50R20LT

Flotation sizes use an older, inch-based format with LT at the end. In 35×12.50R20LT:

• 35: overall tire diameter in inches.
• 12.50: section width in inches.
• R: radial construction.
• 20: rim diameter in inches.
• LT: light truck tire.

Because the diameter is part of the size, flotation sizes make it easy to see how tall a tire is when you fit bigger tires to a lifted truck. They also reach their maximum load at lower pressures than LT-metric sizes with the same load range letter. In the TRA flotation table, a 35×12.50R20LT in load range E reaches its 3,195 lb maximum (load index 121) at 65 psi, while the load range F version reaches 3,640 lb (index 125) at 80 psi. Before fitting a flotation size, have a tire shop confirm that it suits your wheel width, clears the body and suspension, and carries at least what the original tires carried at the pressure you plan to run.

Load index and speed symbol cheat sheet

Load index numbers follow a fixed scale, and each speed symbol stands for a set top speed. The table puts common values side by side; each row pairs two separate scales, not one tire.

The load figure is the most one tire can carry at the pressure its standard sets for that maximum, not at whatever pressure you happen to run. Speed ratings come from laboratory tests. They are not a safe driving speed, and tire makers say they don’t apply to a tire that is underinflated, overloaded, worn, damaged or altered.

Reading the DOT code and the build date

Look on the sidewall for the letters DOT followed by a string of letters and numbers. That string is the Tire Identification Number, and federal rules require its last four digits to be the date code. The first two digits are the week of manufacture and the last two are the year, so 2524 means the 25th week of 2024. Week 01 is the first full calendar week of the year, with weeks counted Sunday through Saturday.

The full number, including the date, only has to appear on one sidewall, so if you can’t find it, check the other side. Knowing the build date helps you spot an old tire before it goes on the vehicle and keep track of your set’s age later.

Match the placard, then shop by size

Tire makers say replacement tires should meet or exceed the load index and speed rating the vehicle maker specifies. The usual exception is winter tires, which can carry a lower speed rating; if you fit them, keep your speed below the rating on the tire. Because load capacity depends on pressure, ask a tire professional to check the load tables if you change sizes or switch between P-metric, Euro-metric and LT tires.

Inflate to the placard pressure, not the maximum pressure printed on the sidewall. The sidewall number is the tire’s limit, not the recommendation for your vehicle.

With your size, load index and speed symbol written down, pick the category that fits your vehicle: passenger car tires, SUV and crossover tires or light truck tires. For year-round driving where winters are mild, start with all-season tires.

When your tires arrive, compare the size and service description on the sidewall with your notes before anyone mounts them. Returns are accepted within 30 days of receiving the order for items that are unused, undamaged and in the original packaging, as the Return and Refund Policy explains.