Cracking the Secret Tire Code

Have you ever been told to read the tire pressure off of the sidewall before filling the tire with air and noticed lots of codes? This article will help you crack the codes and understand the dimensions and performance standards of the tires you drive on.

All tires have a Department of Transportation number indicating the tire passed all minimum DOT standards for sale in the U.S.

DOTAfter the “DOT,” the first two digits are the manufacturer and plant codes. The next two digits are the tire size code. The next three digits are the manufacturer ID code. The last four digits are the week and year the tire was manufactured. For instance, the number 3216 means the tire was manufactured in the 32nd week of 2016.

The tire size code starts with a “P” for passenger car tires, an “LT” for light truck and SUV tires, and a blank for European tires.

MeasureThe number that follows is the width of the tire in millimeters. A P225 is a passenger tire that’s 225 millimeters wide.

Sidewall Hieght: After the forward slash symbol is the aspect ratio of the sidewall height (the percentage of the tire width). A 60 after the P225 would mean the sidewall is 135 millimeters tall.

Tire Speed Rating: The tire speed rating follows the aspect ratio with a single letter. If it is a “Z” it means the tire has the highest tested rating and will have an additional letter at the very end of the tire size markings that reveals the detail of the tire speed.

measure_zMany times a “Z” will have at the end of the tire size markings a “w” or a “Y” to represent speeds of a 168 mph or 186 mph. The below chart shows what speeds the letters represent.

Screen Shot 2017-04-04 at 4.20.35 PM

* Any tire with a speed capability over 149 mph can be listed as a “Z”.
** Although the tires have no upper speed limitation listed, the tires have limited rated speed capabilities.

The testing for the speed chart measures high-speed tire durability under controlled test conditions. These tests do not take into account under-inflation, tire damage, road conditions, all of which could lead to sudden tire failure or loss of vehicle control at lower speeds than what the tire is rated at. The letter “M” indicates that the tire can handle speeds up to 81 mph under test conditions. The more severe the road conditions, the slower the car must be driven.

Construction: The next letter represents the type of construction used to make the tire.

MeasureAn “R” is the designation for radial tires. Radial tires have layers of fabric whose cords run at right angles to the circumference of the tire and whose tread is strengthened by additional layers around the circumference making it the most common type of automotive tire.

Wheel Diameter: The wheel diameter is next and measured in inches. A “16” inch diameter notes that the tire will fit a 16” wheel. A space or gap follows.

Load Index: The next number is the Load Index number that indicates the load-carrying capacity of the tire. The load capacity includes passengers and items in the trunk. The load index assigns a number that corresponds to the maximum load-carrying capacity of the tire. For instance, if your tire is rated with a “97,” the chart indicates a load-carrying capacity of 1,609 lbs. at maximum inflation pressure.

Screen Shot 2017-04-04 at 4.19.57 PM

Some tires don’t use the load index number and instead publish the maximum load number on the tire sidewall.

LoadFor example the words “MAX. LOAD 580 kg. (1279 LBS)” puts the carrying capacity at 1,279 lbs.

Some tires have additional notations or symbols. A diamond at the end of the DOT code means that the tire was manufactured in the 1990s or 1980s.


In another spot on the tire might be a M+S notation, which represents that the tire is good for mud and snow.

© 2017 Hi-Tech Automotive Specialists, Inc.

About CJ Powers

CJ is an author and speaker.
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