Summary: Do you have the right tires?  Government ratings, ratios, PSI requirements, are all displayed in the numbers printed on the sides of your tires.

If you’ve never had to put air in your tires then I congratulate you, but if you’re like me and millions of others, at some point you’ve had to pull into a gas station and pay for use of a compressor so you could re-inflate a sad tire (or three). And, if you’re like me, you’ve probably wondered what all of those letters and numbers on the sidewall mean.

Hopefully you knew that “psi,” meant “pounds per square inch” and that it is the max you should inflate your tires to so that it doesn’t explode in a fatal – albeit theatrical – manner. But what about the rest of it all? Is there anything important? Anything worth knowing?

Typically, a tire will have the manufacturer’s name prominently displayed across the sidewall. You’ll also find a set of letters and numbers that look something like P185/60R 14 82H. Now, the first time I looked at those letters and numbers I had pretty much decided that they weren’t meant for me. They looked far too cryptic. Clearly they were meant for some in-the-know tire guy. But not so! They were meant for me. They’re meant for you, too. And they’re good to know because if you need a spare it’s helpful to have all the info about what size and type of tire you need. And it’s all there in that cryptic set of letters and numbers.

The first letter “P” designates this particular tire as one for a passenger vehicle (there’s also “LT” for “light truck” and “T” for “Truck” among others). The number immediately following it, in this case “185” is the tire’s width in millimeters.

The number after that is the tire’s aspect ratio. The tire’s height is measured from its bead at the rim to the top of its tread. That number is then divided by its width. The smaller the aspect ratio the wider the tire is in relationship to its height. High performance tires usually have a lower aspect ratio than other tires as they have the ability to hug the road when taking turns at higher speeds.

The stand alone letter following the aspect ratio indicates how the tire was constructed. Most often you’ll see the letter “R” indicating that the tire is a radial tire which is the most common type of tire on the road today. You might see the letter “D” for “diagonal bias” or “B” for “Belted Bias” construction. The last number in this series is the size, in inches, of the rim diameter this tire was designed for.

I’d like to say we’re done, but not quite yet. There’s a few more important numbers to learn, but you’re getting smarter by the minute so read on.

Closer to the rim you’ll see another, smaller set of numbers. The first is the maximum load that particular tire can bear in both kilograms and pounds. Following that is the maximum pressure in pounds/square inch to which you can inflate that tire. Important: you don’t want to push either one of these ratings – ever. I know it sounds like common sense, but we all know someone without any sense at all.

On the side of your tires you will also find tread wear, traction, and temperature ratings. Tread wear ratings come from government run tests. The higher the rating the longer the tire can be expected to last. Traction is tested by measuring a tire’s ability to stop on paved, wet ground (both asphalt and concrete are used). The best rating is “AA,” but there are also “A”, “B”, and “C” ratings. Finally, temperature ratings are either “A”, “B”, or “C.” But remember, temperature ratings are performed on tires that are neither over inflated or under inflated, are in good working order with a solid amount of tread, with the vehicle traveling at an appropriate speed which is not excessive, and not carrying too heavy a load. Violate any of these and all bets are off.

And speaking of traction and temperature, remember back in 2000 when Bridgestone-Firestone had to recall all of their Firestone Wilderness XT and Radial AXT II SUV tires? Well, they had a traction rating of “B” and a temperature rating of “C.” Now that you’re in the know, would you buy tires with ratings like those?

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