by Daniel J. Gansle

Created on: August 26, 2008

Last week I was forced to purchase a new set of tires for my still-running 1996 Chevrolet Cavalier. I guess it's a good thing since the old ones were all worn to the bone, dry-rotted, and the tread on one of the tires was literally peeling away. Gazing at the selection in my local Sam's Club Tire & Auto Center, I selected a set of BF Goodrich Tour T/A Pros. A good choice; my ride is now much smoother and they handle like a charm. But what about those numbers? What exactly do they mean?

Deciphering the numbers on the sidewall of the tire may be confusing at first glance, but they provide important information about the tire's dimensions, load capacity, intended purpose, internal construction, and durability at high speeds. Let's break it down using the example of my '96 Cavalier's tire size: P195-70-R14 90T.

Service Type
The majority of tire sizes start with a letter that identifies the type of vehicle and/or type of service for which they were designed:

P = P-metric or Passenger. The most common service type, this signifies that the tire is fitted on passenger vehicles: minivans, cars, SUVs, and light trucks.

T = Temporary Spare. Yep, it's the diminutive spare tire hanging out in your trunk.

LT = Light Truck-metric. This signifies that the tire is fitted on trucks that are capable of towing trailers or carrying heavy cargo: full-size vans, large SUVs, and pickup trucks.

ST = Special Trailer Service. This tire is only used on boat, utility, or car trailers. They are not designed to be fitted on motor vehicles.

Section Width
The three numbers after the first letter signify the tire's width in millimeters. For my tires, that's 195 millimeters if you're measuring across widthwise from the outer sidewall to the inner sidewall.

Sidewall Aspect Ratio
The two numbers after the section width identify the tire's profile (aspect ratio). For my car, that would be 70. What's an aspect ratio, you ask? Here's where it gets technical: 70 indicates that the sidewall height (from rim to tread) is 70 percent of its section width. What does this equation tell us? Higher numbers mean taller sidewalls, and lower numbers mean shorter sidewalls.

Internal Construction
The letter after the two numbers representing the sidewall aspect ratio signify the internal construction of the tire. For my trusty Cavalier, the R means radial, the most common tire construction. However, a D would mean diagonal construction which is used mainly for applications in spare tires and light trucks.

Tire and Wheel Diameter
The two numbers after the letter identifying the internal construction indicate the diameter of the wheel in inches. My tire and wheel diameter is 14 inches. However, many new compact and even subcompact cars today are moving up to the more luxurious 15-inch tires.

Service Description
The last set of numbers followed by a letter indicate the tire's load carrying capacity index (maximum weight on the tires) and speed rating (how fast the tires are designed to go). For my car, that is 90T:

90 = 1323 pounds max (600 kilograms)
T = 118 MPH max (190 km/h) for passenger cars

The service description information is complex, so for more information see the following website: http://www.tirerack.com/tires/tiretech/techpage.jsp? techid=35.

Next time you purchase a new set of tires, impress the service technicians by rattling off your tire size. Then comes the fun part; picking out the tire that best suits your needs.

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