(Last Updated On: February 4, 2021)
The “P” AND “LT” Letters
You can see the letter “P” for most vehicles before the number series begins: P225/70R16 91S. The “P” is short for “P-metric,” which is a classification for a form of tire by the Tire and Rim Association for a “passenger car.” They have equipped this means to be used exclusively for passenger vehicles, including vehicles, minivans, SUVs, and other medium-duty pickup trucks. If you see “LT” instead of “P,” it’s because you need “light truck” tires-“LT” is short for “LT-metric,” which is a designation for a “light truck” style tire by the Tire and Rim Association. Light truck tires are designed for use on vehicles capable of transporting or pulling trailers with heavy cargo.
Similarly, “T” is for your spare tire and stands for “temporary.” That means “special trailer” if you see “ST.”
The width, in millimeters, of the correct tires for your car is the first number to appear in your tire size information: P225/70R16 91S. The measurement from one sidewall to another often applies to the width of the rubber. Therefore, a tire measuring ‘P225’ is intended for a passenger car and has a nominal width of 225 millimeters.
Ratio Of Aspect
The next number you see after the slash mark is for the tire’s aspect ratio, which informs you how tall the profile of your tire is: P225/70R16 91S. Aspect ratios are given in percentages. By dividing a tire’s height from the rim by its width, tire makers measure the aspect ratio. If a tire has an aspect ratio of 70, 70 percent of its width is the height of the tire.
Lower aspect ratio tires, such as 60 series, typically provide performance advantages over higher aspect ratio tires, such as 75 series, for vehicle handling.
The aspect ratio is accompanied by a letter indicating the type of internal construction that preserves the stability of your tire: P225/70R16 91S
On the sidewall of a tire, there are two types of construction that you can see:
- R-The Radial
- D-Diagonal or Ply with Bias
The vast majority of tires on the road in the United States today reflect radial tires; therefore, “R” is usually seen in the tire size classification. Radial construction implies that the tire’s inner ply cords are oriented in a radial direction, basically perpendicular to the axis of rotation, from one bead over to the other.
The next number is the diameter code of the rim on which it can place the tire in inches. A tire with the P225/70R16 91S, for example, would match a 16-inch-diameter boundary.
View other sizes for standard diameters:
- Diameter of 14-Inch
- Diameter of 15-Inch
- Diameter of 17-Inch
- Diameter of 18-Inch
- Diameter of 19-Inch
- Diameter of 20-Inch
- Diameter of 21-Inch
- Diameter of 22-Inch
The next figure in the series is the load index of your tire, which tells us how much weight the tire will handle when fully inflated, in pounds: P225/70R16 91SS
We call it the “index” load because the number doesn’t tell us, at least not by itself, the exact number of pounds the tire can bear. The number does, however, correspond to the unique load capacity referred to in the index. In the load index, numbers beginning with one and ending with 150 reflect carrying capacities of 99 to 7385 lbs.
The final figure is the speed rating in a tire size series, indicated by a letter: P225/70R16 91S. Just as your load index number corresponds to a specific load, your speed rating letter corresponds to a particular capability of speed based on a standardized laboratory examination.
A tire with a speed rating of S is rated up to 112 mph, for example, while a tire with R’s speed rating is rated up to 106 mph. Note that this is not the recommended speed for cruising. On roadways, of course, you should still meet legal speed limits.
Tires with higher speed ratings tend to provide improved efficiency in handling. To retain vehicle speed capability, new tires must have the same or higher speed level. If a car has tires with different speed ratings, the vehicle’s top speed is determined by the speed rating of the’ slowest’ tire.
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