Tires are often unsung heroes of the automobile. If you really think about it, they have an enormous responsibility. They put all the power from the engine to the ground, they do all the steering, and all the braking. You could have a million horsepower, but without a decent set of rubber, all that power just turns into a pile of rubberish sand and a huge cloud of smoke. Your brakes could be the size of your grandmothers best serving dish, but if your tires can’t put that stopping power on the pavement, those gigantic brakes are nearly useless. So what’s in a tire? More specifically to this post, what is on the sidewall of a tire? There are many, many parts to a tire and much more engineering and science than I can explain here, so we’ll just stick with the sidewall gibberish today.

A decent number of people could identify the size of the tire (even if they don’t necessarily know what it all means), and a few could probably even tell you the load range (mostly the truck guys), but what about all the other stuff? There’s a small book written on the sidewall of every tire and it’s important to know what it all means.

First, let’s start with the size.

Take P225/60R16, a fairly common size.

P stands for “Passenger Car”

225 measures the section width or the width of the tire tread in millimeters. The tire tread is the part of the tire that actually touches the road.

60 is the aspect ratio. This number represents a percentage of the section width, in this case 60% of 225 mm = 135 mm.

R indicates a radial construction. We may discuss that in a later post, but for now just know that nearly all automotive and light truck tires are radials.

16 indicates the wheel (rim) diameter in inches. Why they measure the section width in mm and the wheel diameter in inches, I’ll never know, but that’s how it is so don’t question it 🙂

That’s the metric sizing broken down for you.

Then there’s standard sizing in inches.

This is usually reserved for larger “flotation” tires used off road or all terrain truck tires.
31×10.50×15 is really pretty simple.
31″ tall
10.5″ wide (section width)
15″ rim diameter

After the tire size there is a load range and speed rating. 

The load range, or load index, specifies how much weight an individual tire can carry.
The speed rating specifies the maximum safe speed at which a tire can be continually driven.
97T is pretty average for a typical passenger car tire. Click this link for a load range chart and this link for a speed rating chart.

Using the information above we can deduce that a tire with a load range of 97 can carry 1609 lbs. (per tire) for a total weight carrying capacity of 6,436 lbs for all four tires. That same tire is also rated for continued driving speeds up to 118 mph. This is why it’s so important to follow the load index and speed ratings of replacement tires. If you have a V rated tire on a sports car, make sure the replacement tires are also V rated even if you aren’t planning on going 140+ mph to ensure proper handling and performance.

Most tires also have a UTQG (Uniform Tire Quality Guide) rating so the consumer can compare apples to apples so to speak. 

These three ratings are tread wear, temperature, and traction. Check here for a detailed explanation of UTQG ratings. Actually, Tire Rack is a fantastic source for everything tires from buying, to installation centers, to learning about tires for yourself. I’m not affiliated with them in any way, but I’ve spent waaaaaay too much time on their website browsing tires and reading reviews for all of my various vehicles.

All tires approved for use on American roadways have a DOT (Department of Transportation) number on them certifying that they are safe for use on our roads. 

These numbers include lots of information, but perhaps the most interesting bit is the last four digits. Most, but not all, DOT numbers indicate the week and year the tires were manufactured. This is helpful to a consumer because you can make sure you aren’t buying 3 tires that are 4 months old and one tire that is 4 years old. This isn’t common, but it does happen. For example, a tire with the last 4 numbers 5207 means those tires were manufactured in the 52nd week of 2007.

Last, but not least, most tires have seasonal ratings. 

This could be an “M/S” rating for mud and snow, a snowflake for snow, rain for wet weather, or a sun for dry weather. This is important because not all tires are rated for consistent wet weather performance. There are a number of high performance tires on the market that, while DOT approved, are not suitable for regular wet driving.

Well, I hope this lengthy post has helped you understand a little more about the hieroglyphics on your tire’s sidewall.

Bonus: The maximum psi listed on the sidewall of the tire isn’t the recommended pressure. It is simply the highest pressure that particular tire can safely endure to provide maximum load carrying capability. The higher the tire pressure, the more weight it can handle. For the correct tire pressure, consult the tire information sticker usually located in your driver’s door jamb or your owner’s manual.