Deciphering the tire codes

There are several key safety and performance features that everyone shopping for new tires should consider, including tire size, type, speed rating, and load carrying capacity. The problem is that much of that information is conveyed in a series of alpha-numeric codes on the sidewall of the tire or on the vehicle placard and require some deciphering to understand. As someone once said to me, "Where else in the world can you find a mixture of letters, numeric codes, metric and English numeric units, and percentages all mixed together to describe one thing?"

An ancient codex and globe-trotting adventure isn't needed to crack the codes found on your tires, just this clear guide.

Using the tire size P205/55R16 94V as an example:

The P (P-metric) or LT designation at the beginning of a tire size designates whether it is a Passenger or Light Truck size. Keep in mind that many SUVs and even small pickups will be equipped with tires carrying a P designation. LT designated tires are typically found on heavier duty pickups intended for heavier loads.  Tires developed under European specifications (Euro-metric) typically don't carry either designation.

The overall width, 205 in our example, is the cross-section width of the tire measured at its maximum point on the sidewall and is expressed in millimeters (mm).

The aspect ratio, 55 in this case, is the ratio or percentage of the sidewall height to the tire's overall width.  Here, the sidewall is approximately 55 percent of the width of the tire.  Tires with lower aspect ratios typically have shorter sidewalls. 

The R shows that the tire is of radial ply construction -- common to nearly all tires for on-road use today.

The wheel size in our example, 16, is the diameter to which the tire is applied, expressed in inches.

The service code is the combination of the numeric load code (94) and letter speed-rating (V) following the tire size. The numeric portion corresponds to the maximum load-carrying capacity of the tire in pounds. In our example, 94 = 1477 lbs. That same maximum load in lbs. is also listed separately on the sidewall, though often in much smaller print.  This is the maximum load the tire is capable of carrying at its maximum inflation pressure. 

The speed rating is a letter code that designates the maximum speed that tire can sustain as determined by a standardized high speed test protocol.  From our example, V= 149 mph. Though technically this is a measure of a tire's ability to dissipate heat at high speeds, it also, in reality, often represents a tire's level of handling and responsiveness. The higher the speed rating the more responsive and the higher levels of dry and wet grip you potentially find.  Conversely, the higher the speed rating the quicker a tire may wear and the less winter grip you'll have.

When replacing tires you should maintain the same or higher load index and speed rating as the tires originally applied to your car. To learn more, visit the complete tires section at ConsumerReports.org.

--Gene Petersen