Understanding the Markings on Your Automobile's Tires

Posted: Mar 13, 2013
Author: Country Town Tire & Auto Service Center

When we look at the outside of the tire, called the sidewall, we will notice an array of numbers and letters indicating measurements, ratings, and gauges. All of these markings tell us something important about our tires, and it's important to be able to interpret them correctly. How do you know to what maximum speed your tires are rated, or even what type of vehicle they should be on? This could come in handy when buying a used car for example: it's possible the seller has replaced a tire with an unsuitable one, say from a passenger car to a light truck. Though some of the indications are fairly technical, you will be able to decipher these markings more clearly after reading.

• Tire Width – This will be a three digit number, such as 245, and is a measurement in millimeters of the tire width from sidewall to sidewall when the tire is on the rim.
• Aspect Ratio – This is a measurement of tire height given as a percentage of the tire width. For example, a 75% aspect ratio for a 245mm wide tire is 183.75mm tire height. The higher the aspect ratio, the narrower the tire.
• Construction – How the tire was made will be indicated by a marking of a letter. Most newer tires will be marked “R”, for radial, and some older will be “D” or “B” for diagonal bias or bias belted, respectively.
• Rim Diameter – Will be a measurement indicated in inches, around 30.2, for example.
• Tire Type – Is indicated simply by a letter, such as “P” for passenger vehicle, “LT” for light truck, or “T” for temporary (a spare tire). This is very important to note.
• Load Rating – This is a number that correlates to specific weights which the tires can safely carry at a specific pressure (which will also be indicated). The number will not be the actual weight, but rather a grade such as 105: carrying safely 2039 pounds. See the chart below for all corresponding numbers and loads.
• Speed Rating – This is a letter correlating to a rating which indicates the maximum speeds safely handled. For example, a tire labeled “S” will safely be driven up to 112mph. See the chart below for all corresponding letters and speeds.

Load Rating Chart:

Speed Rating Chart: