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Basic Tire Info

Discussion in 'General Tech Articles' started by pissedoffsol, Jan 12, 2004.

  1. pissedoffsol

    pissedoffsol RETIRED

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    Sep 28, 2002
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    Tire Sizes and Ratings Explained

    By: Brian Cummiskey

    Tire sizes have gotten more complex over the past couple years. Most of us know what a 205/40/17 is, but what the hell is P205/50R15 87S?? We'll take a look at all these numbers and explain what they all designate.

    Since 205/40/17 is really a short hand, we are going to take a look at this later tire size: P205/50R15 87S. Let's start our way from left to right


    The P is easy. It simply designates that the tire is a passenger car tire.


    This number designates the width of the tire in mm. This is the actual contact patch that sees the road.


    This number is the profile of the tire. This number is a percentage, and is never a static number. It is based on the width of the tire and is the height of the sidewall. So, the side wall of this tire is 50% of 205mm, or 102.5mm.


    R simply designates that the tire is a steel-belted radial, and not a bias-ply.


    This is the wheel/rim size, measured in inches.


    This is where things get more complicated. This number set represents the tire’s service description. A service description identifies the tire’s load index and speed rating. Service descriptions are required on all speed rated (except for Z-speed rated tires, which we will talk about later) tires manufactured since 1991. The first two digits (87) represent the tire’s load index and are followed by a single letter (S) identifying the tire’s speed rating. The higher the tire’s load index number, the greater its load carrying capacity. Here are some common load index ratings.

    <td>Weight in lbs</td>

    These numbers have a much broader range, ranging from 71 with 761 lb capacity all the way up to 110 with 2337 lb capacity. A tire with a higher load index than that of the OEM tire indicates an increase in load capacity. A tire with a load index equal to that of the OEM tire indicates an equivalent load capacity. A tire with a lower load index than the OEM tire indicates the tire does not equal the load capacity of the original.


    The S denotes the speed rating of the tire. There are several ratings for speed as well as load.

    <td>Character</td><td>Speed (MPH)</td>

    Z-rated tires are the only tires that have a + next to them. They are denoted as 'plus' because while they are only tested to withstand 149 MPH, they 'should' hold for faster speeds. In addition, the Z-rated tires are the only ones to brand its speed rating in the tire size, as in P205/50ZR16. All other speed ratings are shown in the tire’s service description as in P205/50R15 87S.

    When Z-rated tires were first introduced, they were thought to reflect the highest speed rating that would ever be required. Since then, it has proved to be necessary to add W- and Y-speed ratings. (indicated in the tire’s service description) to identify the tires that meet the needs of new vehicles that have extremely high, top speed capabilities. These tires are still referred to as Z-rated in the tire name, but the speed rating and load figures are appended to the end. For example, P225/50ZR16 91Y is a Y-rated tire, even though the Z is still in the tire name.

    Hopefully this will help you understand better exactly what the tire ratings mean. And remember, don't try to do 140 on tires that aren't rated for it: it's just not safe.

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