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Discussion Starter #1 (Edited by Moderator)
Offset and Backspacing Made Simple:

This is an area that can easily cause much confusion. While these terms are often used interchangeably, they are not the same. Further confusing all of this is the fact that rim width is the distance between the rim flanges (where the tire bead contacts the rim), so that the actual wheel width from edge to edge is 1.0" greater that the stated wheel width, making an 8.0" rim actually measure 9.0" wide at it's widest point.



  • Backspacing: refers to the distance from the mounting surface of the wheel (where it contacts the hub) to the back side (inwards-facing) edge of the rim. The easiest way to measure this is by placing a bare wheel on the floor upside down. Place a straight-edge across the wheel, then use a ruler to measure the distance from the bottom of the straight edge to the mounting surface of the rim. Stock fifth generation El Camino wheels have about 4" of backspacing, and this is a good measurement to shoot for. You can try to measure backspacing with a tire mounted on the wheel, but you will have to adjust your measurement if the tire sidewall extends beyond the rim.
Offset: refers to how much the hub mounting surface of the wheel is offset from the wheel's center, usually given in millimeters. Wheels can be positive or negative offset. To get this measurement requires a bit of basic math. First measure overall wheel width from the outside edges of the wheel lips, then divide this measurement in half. Next measure the backspacing as above. An 8" wide wheel is actually 9" wide from wheel lip to wheel lip. If the hub mounting surface were exactly in the center of the wheel, it would have zero offset. It would also have 4" backspacing. Many of the 15" x 8" American Racing wheels are just like this. If the backspacing on this wheel were 3", then the wheel offset would be +25mm (1 inch = 25.4 mm). As wheel diameter and width increase, you can usually get away with a bit more positive offset, as the 15" and 16" Panasports are +22mm offset. Most FWD cars use wheels that range between +30mm to +45mm offset.



Note: Using improperly spaced wheels may require the use of wheel spacers between the hub and wheel mounting surface for proper fitment, but keep in mind that this may require longer wheel studs. Furthermore, wheels that have significantly less backspacing or negative offset than stock (i.e. deep-dish wheels that "stick out") can put extreme stresses on wheel bearings and suspension components. These wheels may also increase steering wheel kick-back. Of course it is always better to get wheels that fit correctly, and not have to use spacers at all.


  • Deciphering Tire Sidewalls:




Before buying new tires, take a minute to learn what all those numbers and letters on the tire sidewall mean:

  • Tire Size:

    Let's say, for example, we have a P185/60-R15 tire. The "P" indicates this is a passenger car tire. Many times this is omitted (LT = Light Truck tire). The first number is the Section Width of the tire in millimeters, measured from sidewall to sidewall of a fully-inflated tire without any load placed on it. For a 185/60-15 tire, the width is 185mm or 7.28" (to convert to inches, divide by 25.4). It is important to note that this is not the tread width of the tire at the road surface. The second number is the Aspect Ratio, a ratio of sidewall height to tire width (a percentage of the section width). The tire above is 7.28" wide, so we multiply by the aspect ratio to find the height of one sidewall. In this case, 185 x 0.60 = 111mm (or 7.28" x 0.60 = 4.36"). The "R" means this is a radial tire. The last number is the Diameter of the wheel in inches. When switching to larger diameter wheels, try to keep the tire outside diameter the same as the car's stock tire to avoid problems with speedometer and odometer calibration and changing overall gear ratios.
    To calculate the outside diameter of a tire: Take the sidewall height and multiply by 2, (remember that the diameter is made up of 2 sidewalls, one above the wheel and the one below the wheel) and add the diameter of the wheel to get your answer.
    For example: 185/60-14: 185mm x .60 = 111mm x 2 = 222mm + 355.6mm (14") = 577.6mm or 22.74" diameter.
    Note: When mounted on a wider than designed wheel, tire section width increases by 0.2"(5mm) for every 0.5" increase in rim width. This would make a 185mm wide tire mounted on a 7.5" rim have a section width of 190mm.

    A general rule of thumb for a high-performance tire is a rim width that is 85-90% as wide as the tread of the tire (not tire section width).
  • Speed Rating:

    The maximum safe top speed of a tire under perfect conditions is given as a letter in part of the tire size designation. If the tire says 185/60R15 85H or 185/60HR15 on the sidewall, the R indicates it is a radial tire, and the H means it is speed rated up to 130mph. Common speed ratings are:
<center> <table border="1" cols="3" width="69%"> <tbody> <tr align="center" valign="center"> <td>Q=99 MPH, 160km/h</td> <td>U=124 MPH, 200km/h </td> <td>W=168 MPH, 270km/h</td></tr> <tr align="center" valign="center"> <td>S=112 MPH, 180km/h</td> <td>H=130 MPH, 210km/h</td> <td>Y=186 MPH, 300km/h</td></tr> <tr align="center" valign="center"> <td>T=118 MPH, 190km/h </td> <td>V=149 MPH, 240km/h</td> <td>Z=149+ MPH, 240+ km/h </td></tr></tbody></table></center>



  • Treadwear, Traction, Temperature, and Load:

    The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) labeling system to establish ratings for treadwear, traction, and temperature resistance. These tests are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. This is designed to tell the consumer how long a tire will last, the distance needed to stop, and how quickly the tire heats up. Since each tire manufacturer interprets their tests in their own manner, this makes comparing two tires from different manufacturers virtually impossible, so take these ratings as only a very rough guide. While traction and temperature resistance ratings represent specific performance levels, the treadwear ratings are assigned by the manufacturers following field testing and are most accurate when comparing tires of the same brand.
  • Treadwear:

    Treadwear receives a comparative rating based on wear rate of the the tire in field testing following a government specified course. For example, a tire grade of 150 wears 1.5 times longer than a tire graded 100. The tested tires are only worn down part way, so this rating can't really say anything about total tire tread life. Actual performance of the tire can vary significantly depending on conditions, driving habits, care, road characteristics, and climate, so take these ratings with a grain of salt. Ratings are valid for comparison of tires made by the same manufacturer, but not for tires made by different manufacturers. DOT-approved auto-x competition tires typically have treadwear ratings from 0 to 60. Ultra sticky high performance street tires have ratings in the 120 to 180 range. Touring tires can have treadwear ratings above 400.
  • Traction:

    Straight-line wet braking traction is represented by a grade of A, B, or C, with A being the highest rating. In 1997 a new top rating of "AA" was introduced to indicate even greater wet braking traction. However, due to its newness, this grade will probably be applied initially to new tire lines as they are introduced and later to existing lines which excel in wet braking, but had been limited to the previous top grade of "A". Traction grades do not indicate wet cornering ability.
  • Temperature:

    Temperature resistance is graded A, B or C, with A being the highest rating. It represents the tire's resistance to the heat generated by running at high speed. Grade C is the minimum level of performance for all passenger car tires as set under Federal Motor Vehicle Safety Standards. This grade is established for a tire that is properly inflated and not overloaded. Some competition auto-x tires are designed to have a B or C rating so that they heat up enough to reach full operating temperature early on in an auto-x course.
  • Load Ratings:

    Many tires come with a service description added on the end of the tire size. These service descriptions contain a number, which is the Load Index, and a letter which indicates the speed rating. The load index represents the maximum load each tire is designed to support. Because the maximum tire load capacity is branded on the tire's sidewall, the Load Index is used as a quick reference. Multiply the tire load rating by 4 to get the maximum carrying capacity of the tires. Never use a load rating below that specified by the car's manufacturer. See the tires' Load Chart for specifics.
  • Date Code:

    Every tire has string of numbers and letters molded into the sidewall that tell the tire serial number, DOT compliance code, and when the tire was made. This information is usually found near the rim, and often has a flat-head screw imprint both before and after it, as the information is stamped into a plate that is screwed into the tire mold. Look at the last three numbers in the code to get the week of manufacture and the year. If the last three numbers were 127, this would mean the tire was made the in the 12th week of 1997 (or 1987, for that matter). Why would you want to know this? Well, if you're buying a car and the owner says they just put on new tires, but they don't look that new, check the date code. Be aware that you can buy new tires that may have been sitting in a warehouse for a few years, but the date code is usually less than 2 years old for new tires.
Competition Tires:

For those interested in a street-legal tire that gives near-racing slick grip, consider a set of DOT legal competition tires. These are not for winter use, may not work well in the rain, and often wear out in 5,000 - 25,000 miles (or less), but their performance capabilities will make you smile. Treadwear ratings (for what they're worth) will range from 0 to 60. Choices include both road-racing and auto-x tires, with the auto-x tires often having a softer compound and a carcass that heats up faster than a road racing tire. Popular choices among autocrossers and road-racers include the BFG R1, BFG G-force, Yokohama A008RSII, Yokohama AS032, Toyo Proxes, Kumho V-700, and Hoosier AS302/RS302. To makes these tires last longer, it is important to heat them up to full operating temperature the FIRST time they are used, and then let them cool and rest for at least 24 hours to "cure" the rubber compound. Tires treated this way can last twice as long as non-heat cured tires.
Article contributed by: ElkyPete
 
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