Reading Your Tire Sidewall: The Numbers and Letters Explained

Breaking Down the Code: What Each Segment Means

Take a tire marked P215/65R17 98H. That string contains at least six distinct pieces of information. Here's what each one tells you.

Service Type (P, LT, ST, T)

The optional letter before the numbers indicates the tire's intended vehicle class. P stands for passenger vehicle. LT means light truck. ST is for special trailer use, and T denotes a temporary spare. Some tires carry no prefix — these are built to European "Euro-metric" standards and have slightly different load-capacity calculations.

Section Width — The First Number

The three-digit number (215 in the example) is the tire's section width in millimeters, measured across the tire's widest point when mounted on a specified rim. A wider number means more rubber contact with the road.

Aspect Ratio — After the Slash

The two-digit number following the slash (65) is the aspect ratio — the sidewall's height as a percentage of the section width. So a 215/65 tire has a sidewall height of about 140 mm (65% of 215). Lower aspect ratios (40, 45) mean shorter sidewalls, which generally improve cornering stiffness but can produce a harsher ride.

Construction Type — The Letter After Aspect Ratio

R stands for radial construction, which is the overwhelming standard for modern passenger tires. You may occasionally see D (diagonal/bias-ply) on older vehicles or specialty trailers.

Rim Diameter — The Last Number in the Size String

The number after the construction letter (17) is the rim diameter in inches. This must match your wheel exactly — a 17-inch tire cannot seat safely on a 16-inch or 18-inch rim.

For related maintenance context, see our article on tire rotation, balancing, and alignment — three services that work in tandem with choosing the right tire.

Load Index, Speed Rating, and the DOT Code

Load Index

The number after the rim diameter (98 in our example) is the load index. Each number maps to a maximum weight capacity per tire. Index 98 equals 1,653 lbs. You should never install tires with a load index lower than your vehicle's door-jamb placard specifies — doing so risks structural tire failure under normal load conditions.

Speed Rating

The final letter (H) is the speed rating — a design specification indicating the maximum sustained speed the tire structure can handle. Common ratings include S (112 mph), T (118 mph), H (130 mph), V (149 mph), and W (168 mph). Downgrading your speed rating below the manufacturer's minimum is not recommended, even if you don't plan to drive at those speeds — the rating also reflects heat-management and structural design thresholds.

The DOT Code and Tire Age

Somewhere on the sidewall you'll also find a DOT code — a string beginning with "DOT" followed by a series of letters and numbers. The last four digits are the most actionable: the first two indicate the week of manufacture, and the last two indicate the year. A tire reading 2319 was made in the 23rd week of 2019. NHTSA recommends consulting your vehicle manufacturer's guidelines on tire replacement age, with many automakers suggesting replacement after six to ten years regardless of visible tread depth. Rubber degrades internally over time.

If your dashboard shows a tire pressure warning, see our guide on dashboard warning lights decoded to understand what action is needed. And just as you'd read a nutrition label to make informed food choices — see how nutrition labels work for a parallel look at decoding standardized information — your tire sidewall rewards a few minutes of attention with better-informed maintenance decisions.