Tire sizes, explained

Tires are your car’s point of contact with the road and an essential component to any functional vehicle. The tire sizes and types you use can have different effects on your car’s performance. Race cars, for instance, use slicker tires that minimize friction. Off-road cars, on the other hand, are likely to use much bulkier tires with significantly more tread to maximize traction.

How to read a tire size

Understanding how to read a tire size can be beneficial if you’re looking to replace yours during a regular maintenance service or tire rotation, or if you want to switch them out for performance reasons. Car tires typically come with a series of letters and numbers printed on them, which can tell you several things about the tire at a glance.

Tire type

The first letter in the code printed along the side of your tire will tell you the type of tire it is.

• “P” stands for “passenger” vehicles, which covers most consumer cars like sedans, minivans, SUVs, smaller pickups and crossovers.
• “LT” stands for “light trucks,” designed for vehicles that are intended for heavy towing, such as larger three-quarter or 1-ton trucks.
• “ST” stands for “special tire,” and is used to denote tires for trailers like fifth wheels, travel trailers and other utility trailers.

Occasionally, you may see car tires with no letter at the front. This usually indicates that the tire is a European-sized model, which may have different load capacities than P or LT tires do. Some European tires meant for commercial use may be preceded by a “C.” It’s helpful to check the tire placard and your owner’s manual to ensure that you’re getting the correct tire type for your vehicle.

Tire width

The first three numbers printed on the tire’s sidewall, regardless of whether there’s a letter preceding them or not, indicate the width of the tire, measured in millimeters from sidewall to sidewall. This width is one of the most important tire dimensions to know as a car owner, as it affects grip, comfort, appearance and price.

Aspect ratio

Next, you’ll see a two-digit number (possibly separated from the width measurement with a “/” mark) that indicates the tire’s aspect ratio, another important tire dimension. This is the ratio of the tire’s sidewall height (measured from the wheel rim to the top of the tire tread) to its sidewall width. Note that the two-digit number expresses this ratio as a percentage. For instance, a tire with an aspect ratio of 65 has a sidewall height (sometimes called its “profile”) that is 65% of the tire’s sidewall width.

Construction type

Next up you’ll see a single letter that indicates the tire’s construction, usually either “R,” “D,” or “F.” Here’s what that means:

• “R” stands for radial construction, which is generally considered the standard for most tires these days. This means the internal plies— layers of strong, rubber-coated cords inside your tire — are aligned perpendicular to the direction of travel.
• “D” stands for diagonal construction, sometimes also called bias-construction and indicated with a “B.” This construction style features internal plies that run diagonal to the direction of travel. They’re sometimes layered in opposing directions to one another to create an “x-ply.”
• “F” (or “RF”) stands for run-flat tires that are designed to let you keep driving in the event of getting punctured. It should be noted that these tires aren’t meant to last indefinitely after a puncture. They’re just intended to give you a little time to get the tire properly repaired or replaced.

Wheel diameter

Next, you’ll see a two-digit number indicating yet another one of the tire dimensions — the wheel diameter, measured in inches. This is the size of the wheel rim that the tire can fit snugly onto.

Load index

Following the diameter is a two- or three-digit number specifying the tire’s load index. This number is an indication of how much weight the tire can support. Note that this number isn’t the actual weight itself, but rather a reference code that can be used to look up the weight capacity on a load index. Learning how to read a tire and find its load index can help you ensure you’ll get tires that are appropriate for the weight of your vehicle.

Speed rating

The final letter (sometimes more than one) on your car tire’s sidewall indicates the tire speed rating. Similar to the load index, this is a reference code used to look up the actual recommended top speed in an index. Tires with higher speed ratings will perform better and provide more control at higher speeds.

In summary

Choosing the right tire size for your car shouldn’t leave your head spinning. Consulting with your car’s owner’s manual should tell you what kinds of tires are suitable for your car. From there, it’s just a matter of reading the codes printed on the tire’s sidewall to identify its type, dimensions, construction style and other details to find the ones you need.