A.D.S. AUTOMOTIVE & TIRE
Reading Your Sidewall - What Those Numbers Mean
Understanding Those Numbers
You can learn a lot from the sidewall of your tire. At first you may think you stumbled across some kind of "Code", but actually found molded into the tires are it's specifications.
Example P235/75R15 91W
P Identifies your tire as a Passenger Tire. The P stands for "PMetric". If your tire size starts with LT than it identifies the tire as a "Light Truck" tire.
235 Identifies the tire section width, which is the measurement of the tire from sidewall to sidewall in millimeters. This measurement varies depending on the rim to which it is fitted.
(There are 25.4 millimeters per 1 inch.)
75 is the aspect ratio. This percentage compares the tires section height with the tires section width. For example, this aspect ratio of 75 means that the tires section height is 75% of the tires section width.
R indicates the construction used within the tires casing. R stands for radial construction. B means belted bias and D stands for diagonal bias construction. Most common tires today are of the Radial (R) type.
15 The last dimension listed in the size is the diameter of the wheel rim which is most often measured in inches.
Load Index and Speed Rating
91 is the he load index and speed rating, or service description are the numbers that follow the tire size.
The load index tells you how much weight the tire can support when properly inflated. Load indices range from 74 - 150 for passenger tires with each numeric value corresponding to a certain carrying capacity. The carrying capacity for each value can be found on a load index chart. On each U.S. passenger car tire, the load limit is listed in pounds(lbs.). European tires have the load limit listed in kilograms(kg) and sometimes pounds(lbs.).
W Speed ratings are represented by letters ranging from A to Z. Each letter coincides to the maximum speed a tire can sustain under its recommended load capacity. For instance, "T" is equivalent to a max speed of 118 mph. Even though a tire can perform at this speed, we do not condone exceeding local speed limits.
The Origin of Speed Ratings
We can thank Germany’s famous Autobahn for tire speed ratings. Tire speed ratings range from A (the lowest) to Y (the highest). But the chart is not completely in alphabetical order. For example, H is between U and V, with the common perception that H stood for “high performance” at one time. As manufacturers continue to add speed to their vehicles, tire speed ratings evolve to match the speeds. For example, Z was the highest rated speed at 149+ until W & Y were used to match the higher speeds of exotic sports cars.
DOT Serial Number
The "DOT" symbol certifies the tire manufacturer's compliance with the U.S. Department of Transportation (DOT) tire safety standards.
Below is a description of the serial number. Starting with the year 2000, four numbers are used for the Date of Manufactuer, first two numbers identify the week and the last two numbers identify the year of manufacture.
Prior to the year 2000 three numbers were used for the Date of Manufacture, the first two numbers identify the week and the last number identifies the year of manufacture. To identify tires manufactured in the 90's a decade symbol (a triangle on its side) can be found at the end of the DOT serial number.
|Speed Symbol||Speed (KMH)||Speed (MPH)|