Tires 101: How To Read Your Tire Sidewall
There are a lot of letters and numbers molded into the sidewall of tire, and they’re there to make sure you put the right equipment on your vehicle. Unfortunately, most tires aren’t sold with a vocabulary card that lets you identify what all those marks mean. But if you know what you’re looking at, reading a sidewall and identifying a tire can be a quick & easy process.
Metric or P-Metric Sizing
A tire sidewall showing its size in metric lists the width first, then the sidewall height (shown as a percentage of the tire width), followed by a letter to designate what type of carcass construction, and lastly the wheel/rim size.
Let’s look at the sidewall of this 245/245ZR17 G-Max AS-03:
245 is the section width (mm)
45 is the aspect ratio (%)
ZR with “R” representing radial construction & “Z” showing it’s rated for 149mph +
17 is the wheel diameter (inches)
95 is the load index
W is the speed rating
This G-Max tire is 245mm wide (about 9.6 inches wide), 25.6 inches tall, and fits a 17-inch wheel. It has a carrying capacity of 1,521 lbs and a maximum operating speed of 168 mph. Note that metric sizing uses both millimeters and inches, and requires conversions. Also, the speed rating references the testing and does not imply that the consumer drive at these high rates of speed.
Certain light truck and heavy-duty tires use American (aka American standard, aka imperial) measurement in inches. The tire height (or diameter) is most often listed first, followed by the section width, then the carcass construction type, and lastly the wheel size that the tire fits.
Let’s take this 33x12.50R17 Grabber AT2 for example:
33 is the tire height, or diameter (inches)
12.50 is the section width (inches)
R represents the carcass construction (radial)
17 is the wheel diameter (inches)
105 is the load index
Q is the speed rating
The tire is 33 inches tall, has a width of 12.50 inches, and fits onto a 17-inch wheel. It has a carrying capacity of 2,039 lbs and maximum operating speed of 99 mph.
For American standard sizing, the tire height, or diameter, is listed directly in inches. On a metric tire, you’ll need to do some math. For metric sizing, take the section width and convert it to inches (divide by 25.4), then multiply that by the aspect ratio (shown as a percentage) to get the sidewall height. Next, take the sidewall height, double it, add the wheel diameter, and you’ll have the diameter of a metric tire.
This is shown in either millimeters or inches, but it refers to the overall carcass width, not the tread width.
Most light truck and passenger vehicle tires built today are built with radial construction (“R”). This is shown before the wheel size. There are three basic types you can find produced today:
B Bias belted
D Diagonal bias
Wheel size is shown in inches, regardless if it’s a metric or American sized tire.
The load index shows the tire’s weight carrying capacity. While the load index is a numerical value, it is just a representative symbol for a certain carrying capacity and does not numerically correspond to carrying capacities (Example: Load index 85 indicates a carrying capacity of 1,135 pounds, not 85 pounds or 850 pounds or 8,500 pounds).
In the early 1990s, tires started being labeled with speed ratings, showing the maximum operating speed of the tire. A “Z” rating was given to tires tested to be “safe over 149 mph,” and while the speed rating is usually shown after the load index on a tire, many manufacturers chose to represent the high-speed rating in the tire size with a “ZR” designating a radial construction capable of operating over 149 mph. As performance and technology increased, two more specific (higher) speed ratings were made up: “W” & “Y.”Speed rating Maximum operating speed
Prior to (or sometimes following) the tire size—on both American and metric sizing—a tire may or may not have a letter showing the service type. Most consumers will deal with tires stamped with either “P” standing for passenger vehicle tire, or “LT” standing for Light Truck. But there are a number of others.
Here’s a quick reference guide:
P Passenger tire, often referred to as “P-metric”
LT Light Truck tire
T Temporary tire, or spare tire
ST Trailer tire (Special Trailer service)
C Commercial tireSHARE THIS