Tire sizes will almost always begin with a letter (the P in P205/55R16) and usually appear as the biggest numbers on the tire. In the image above you see 205/55R16 91V. Notice that there’s no P? If you don’t see the letter, don’t worry. All you need to know for now is what you’re looking at; In 205/55R16 you’re seeing the Tire Width (205), Aspect Ratio (55), Construction Type (R), Wheel Diameter (16), Load Index (91), and Speed Rating (V).
But what does the letter mean?
Very simply, “P” stands for Passenger and “LT” stands for Light Truck. Passenger and Light Truck tires are the most common tire types. Unless you’re driving a specialty vehicle you’re only going to see these two letters (or none at all).
Tire Width
The first number after the letter (if there is a letter) tells you the width of your tire, in millimeters. This is the distance from one side of the tread to the other. If you’re the curious type, take a ruler or measuring tape and lay it across the tire; you should see the same number on your measuring tape as is printed on the side of your tire.
Aspect Ratio
Simply put, this is how tall your tire is. In even simpler terms, this is how much sidewall you see when the tire is mounted. Performance tires tend to have a lower aspect ratio while your typical all-season passenger tire will be between 65 and 80.
Radial
That “R” means your tire is a Radial tire. All modern tires are radial tires, with very few legacy exceptions for classic cars or specialty vehicles (like farm equipment). Sometimes you might see “VR” or “ZR” but that just means the tire’s speed rating is V or Z, which we’ll explain more below.
Diameter
This is the diameter of the empty hole in your tire where the wheel goes. In the above you see that the diameter is 16 so this tire goes on a 16 inch wheel. Diameter does not refer to the overall diameter of the tire.
Load Rating
The Load Rating tells you how much weight your tire can hold by itself when mounted and filled with air. For example, the tire above has a 91 load rating and can hold 1356 pounds by itself. Just keep in mind that your max load rating will be multiplied by all the tires on your vehicle. So 1356 pounds x 4 tires = 5424 pound max load.
Speed Rating
Every tire gets a speed rating which is then marked by a letter. The speed rating tells you the maximum speed that your tire can safely reach before it will start to breakdown or be at risk for failure. The above example has a speed rating of V which means it can travel up to 149mph safely.
