All About Tires

A wide open road. Two lanes. Blacktop. The only thing that separates you from the asphalt is your tires, so it’s a really good idea to make sure they are equipped for the job. In order to check, we need to learn how to read their language. There are numbers and letters, and then some more letters and numbers and a few words, but none of it makes much sense unless you know what you’re looking at. So here’s how to read your tires.

Tire Math
In between all the chaos on your tire sidewall, you will need to find your tire size. Apart from the manufacturer’s brand name, the largest font is probably going to be your tire size. For example: 185/60 R 17. The tire size is composed of two numbers. The first one is the width of the tire in millimeters. So 185 means our example tire is about 7 and 1/3 inches wide. The second number after the slash is the aspect ratio. This is the ratio of the tire’s sidewall height to the tire width. In the example above, 60 means the height of the tire is 60% of its width. So a tire that is 7 1/3 inches (185 mm) wide and with an aspect ratio of 60 is about 4 1/3 inches in height which is about 60% of 7 1/3 inches. Now if you’re crossing your eyes already, stay with me here. That is the end of the tire math. Most everything else on your tire is a rating, a code, or a measurement.

Alphabet Soup
The next digit to decode is usually a letter or, on some fancy tires, a combination of letters. This code signifies the construction method of the tire, or how the layers of the tire construction materials lay under the rubber. Most tires sold for passenger vehicles, SUVs, and light trucks are radial construction tires. Hence the R you see on tire sidewalls of most cars you see in the street. Sometimes elsewhere on the tire you will also see a description of materials and how many layers of each material there are, along with where they are, either the wall or tread of the tire. General rule of thumb is the more information you see on the tire, the higher likelihood it is a fancier brand tire, or what the shop might call a “name brand” tire. Some shops might also try to sell you used tires because they are a name brand, even if they aren’t very good. If you don’t want to bother visually inspecting used tires for signs of age and wear, you don’t have to! Just never buy used tires unless you already have a good relationship with your tire shop.

The final number you will see is the measurement of the wheel size or the rim diameter that would fit that tire. The rim is the outer lip of metal that holds the tire onto the rest of the wheel. The number 17 in the example above indicates a 17-inch rim diameter or wheel size. If you were to only remember one number from this entire article to double check for on all your tire shop receipts, it would be your wheel size. Bigger tires, which some larger trucks may require, usually cost more money. The wheel size is a number you will see on your tire shop receipt. This is likely not a number that is going to change over the course of you owning your vehicle. If there is a big difference between what is on your tire sidewall and what is on your receipt, you should very politely look into why that could be with someone nice from the front desk, not a technician.

Time Is On My Side
The other important grouping of symbols to take note of is the Department of Transportation (DOT) code. If you don’t see this code on your tires, they may not be street legal since they may not be made to the specification that the DOT deems safe. The DOT code always starts with DOT. Then there is a group of four letters (or two groups of two letters) followed by a group of three letters, and finally a group of four numbers. You can look up what the other codes mean, but those last four numbers are the first thing you should look at when inspecting used tires you want to buy. The first two numbers tell you the week of manufacture and the last two tell you the year. For example: 5220 would translate to the 52nd week of the year 2020. As good practice, you should retire any tires that are older than six years. No matter if they look like they have good tread, the rubber compounds are likely beginning to break down. This is also a good thing to check and use as a bargaining tool when you are shopping for a used car.

Penny For Your Tread
When shopping for used tires, remember to also check for any rips, cracks, bumps, or other imperfections in the rubber surface of the tire. Notice any uneven tread wear—this usually indicates alignment problems if you’re looking to purchase the car that comes with the tires as well. Finally, check for tread depth by sticking a penny into the middle of the tread to see if it covers Lincoln’s head. If the tread covers any part of his head, you still have some time left on those tires. If you can see the top of the head of our 16th president, those tires are no good. Don’t let anybody sell you overpriced used tires when you know they won’t last.

You’ll notice there may be a lot of other information on your tire that I did not go over. A lot of it is more helpful for tire shops to sell you more expensive tires than what you actually need. Be aware of this when trying to figure out what tires you can afford and still eat. You don’t need to memorize every single number and letter to know what kind of tire you need. All you need to do is look in the owner’s manual or the manufacturer’s door jamb sticker or badge in your vehicle. There you will find the recommended tire size including the tire width, aspect ratio, construction type, and wheel size for your vehicle. This information is good to have on a sticky note in front of you when you call around to tire shops in your area and ask for quotes. Not all shops will stock the most affordable tires in your size, so knowing this information can help you get lots of quotes from local shops easily and quickly. This way you can compare offers, giving you—the informed consumer—the upper hand. If you can’t find or can’t see your original sticker, you can probably find a copy of it online if you know some basic information about your vehicle’s make, model, and engine size.

Under Pressure
The other incredibly important information that sticker or badge contains is the recommended cold tire pressure. This is the pressure reading you want to see on your tires first thing in the morning before driving anywhere. In the U.S., this reading is typically made in PSI (pounds per square inch). In general, compact cars usually require around 30 to 35 PSI while bigger trucks and SUVs can be anywhere from 60 to 80 PSI. Buses and RVs can even get up into the 100s.

Low Maintenance
If you just invested in some new tires, congratulations! That’s a big purchase. There are a few things you should do to protect your investment. Every time you get a new set of tires, ask for an alignment. This is particularly important and worth the money upfront with the state of our roads. Even the most careful driver will get knocked around in these mean, pothole-filled streets. If your alignment is off—and chances are if you drive in this city it is—your tires wear unevenly. This compromises their structural integrity. All this means is that those expensive new tires will wear out a lot faster than they should. Additionally, rotating your tires frequently, every time you change your engine oil or about every six months, will help in maintaining proper alignment and avoiding uneven tread wear. If you aren’t changing your own oil, then ask for a tire rotation whenever you bring your car in to your favorite mechanic. This is an easy service to ask for when you are still trying out different mechanics or shops.

Oh The Places You’ll Go!
If you take care of your tires, they will take care of you. Not enough nerdy tire talk here? I invite you to dig deeper into different wheel and tire fitments people have tried for your vehicle’s make and model. You may discover a new look you like, or maybe a ranking of different tires and how they performed for your particular vehicle. If you start paying attention to your tires, you will notice your car performs better and you’re spending less money at the pump. If you’re trying to pinch pennies to make a small tour happen, or just a road trip, saving a few bucks on gas can really add up. Now this money can be used to buy an incredibly memorable and unique souvenir handmade by a local artist from the place you visited. Much better use than lining  the pockets of gas and oil companies.

Got questions about your car, truck, SUV, or other engine-powered vehicle?

illustrations by Deanna Larmeu