Tire Specifications

by Deb Murphy on November 22, 2008

What Do All Those Numbers Mean?

Everything you need to know about your tires is stamped on the sidewall. We’ll start with the basics and work our way into the center. For a more complete description, keep reading.

  • Size: Light truck sizing starts with “LT;” passenger car tires, with “P.” The letter will be followed by a combination of numbers and letters, i.e. 215/80R16. If you’re shopping for new tires, that’s the information you’ll need.
  • Load rating: Light truck tires will have a letter rating, i.e. “D.” Passenger tires use a double digit number.
  • Speed rating: Passenger car tires use a letter, from M to Y or ZR. Light truck tires’ speed rating also starts at M but stops at H.
  • M+S: If this follows the load or speed rating on your tire, it indicates the tire is designed for severe snow conditions.
  • The manufacturer’s name and tire model name completes the outer circle. Passenger car tires also include treadwear, traction and temperature grades in small print between the manufacturer and tire names.
  • Light truck tires will have two circles of information next to the center. The outer layer includes maximum load and inflation when the tire is used as a single; the same information when used as a dual and load inflation limits.
  • The inner circle on light truck tires and the only additional “small print” on passenger car tires include: the U.S. Department of Transportation (DOT) identification number, tire ply composition and materials used, maximum load rating and maximum permissible inflation pressure. Your vehicle will have its own tire inflation guidelines included in the owner’s manual as well as printed on the driver’s door jamb.

Under normal circumstances, the easily-read information on that outer circle will be all you need to reference. In the event of a recall or defect problem, the DOT number will be important.

Tire Size

The tire size printed on the sidewall needs some explanation. First, it’s in metrics. The first three digits, using the sample above 215/80R16, “215″ is the section width (official definition: distance between the outer sidewalls of an inflated tire; simple definition: the width of the tread surface). “80″ is the aspect ratio (official: section height divided by section width; simple using our example: the sidewall height is 80% of the tire width or 172 mm). The aspect ratio is used to define a series of tires. The above would be an 80-series tire. The “R” indicates the tire is a radial, which most tires are. The final number is the tire rim diameter which corresponds to your wheel diameter.

While 14- to 16-inch wheels were once the  norm, styling trends today are toward larger wheels (up to 22-inches) and low profile tires (the aspect ratio number is more like 30 rather than the 65 to 85 norm). The tire, basically, looks like a rubber band wrapped around massive wheels. If your vehicle didn’t come equipped with this large wheel/low profile tire package but you’re dead set on looking cool, proceed directly to a tech-savvy tire dealer to make sure the “look” is compatible with your vehicle and that you fully understand any vehicle handling changes.

Sizing for oversized off-road tires is not based on the metric system. The first number, the height or diameter, is in inches: i.e., “30,” “37″. You really need to know what you’re doing before upsizing overall diameter. You’ll need a suspension or body lift to keep the tires from ripping out your wheel wells during sharp turns. There are also gearing ramifications and if you go up more than 10-percent in overall diameter, your speedometer and odometer readings will be off.

Speed Rating

This concept was introduced in Europe where legal speeds can reach 100 mph. The rating is expressed in letters:

  • M: speed limit 81
  • N: 87
  • P: 93
  • Q: 99
  • R: 106
  • S: 112
  • T: 118
  • U: 124
  • H: 130
  • V (VR): 149
  • W (ZR): 168
  • Y (ZR): 186

The ratings were developed to match tire to speed capabilities of the vehicle. Putting a Y-rated tire on a Yugo will not make it go faster. If your vehicle comes with a specific tire speed rating, you should never go lower than that rating; higher is okay.

To withstand the speeds for which they are rated, these high performance tires’ construction is stiffer than conventional, non-rated or low-rated tires.  The trade-off is in ride quality as well as possible changes in handling.

Load Rating

Load ratings come in two forms: load index, the maximum weight a tire can handle vertically, or on top of the tire, generally between 80 (992 lbs.) and 100 (1764 lbs.) for both passenger and light truck tires. This is not a static number. Heat impacts tire performance, as do driving conditions. Sharp cornering, or evasive maneuvers throw more downward weight on the tires. If the load the vehicle is carrying is at or near the limit of the tire, drive cautiously, or consider upgrading your tires.

Load range is an indication of how much horizontal, or side to side, load the tire can handle and is used for light truck tires. For passenger car drivers, horizontal tire stress is difficult to imagine. Cars have a low center of gravity and usually transport people as opposed to cargo. Light trucks, however, have a higher center of gravity and are designed for towing and cargo applications. The tires have to hold up under shifting, side-to-side weights, especially when towing. Load range is associated with ply rating. The sidewalls of the tire have to be stiffer to absorb the horizontal weight. Load range and corresponding ply rating start at A with a 2 ply rating and go up to N with a 26 ply rating. To put this in perspective, the Volkswagen buses have a suggested load rating of C while a full-size pick-up (half-ton and up) will have a recommended load rating of D or E.

 If you have been involved in a tire-related accident, contact us using the form on the right and we’ll help you locate an attorney.

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