How to Read a Tire Sidewall Like a Mechanic in 60 Seconds

The first time someone showed me a tire sidewall up close, I was sixteen years old, standing in my dad’s garage in Sterling Heights. He pointed at a string of letters and numbers on his old F-150’s tires and asked me what they meant. I had no idea. I guessed that “P” stood for “passenger.” I was right about that one. Everything else? Wrong. Twenty-two years later, I still see people staring at sidewalls like they’re written in code. The tire shop prints a quote, the customer nods, and nobody actually reads what’s molded into the rubber. That’s how you end up with the wrong tires. That’s how a guy with a heavy-duty diesel ends up with passenger tires on the rear axle. That’s how a lifted Jeep ends up with the wrong load index. The information is right there, in plain text. Most people just don’t know how to decode it. Let me fix that for you in about sixty seconds. Pick a tire. Any tire. Walk outside to your car. Crouch down next to the front wheel. Look for the string of characters that starts with a letter, then a few numbers, then a slash, then more numbers, then a dash, then two more numbers. It’ll look something like this: P225/60R17. That’s your tire size. It contains everything you need to know about width, aspect ratio, construction, and wheel diameter. The first letter tells you the tire’s intended use. P means passenger car. LT means light truck. ST means special trailer. T means temporary spare. If you have a heavy-duty pickup and you see “P” at the beginning of the string, you have a problem. Those tires aren’t built for the weight or the towing demand. If you have a crossover SUV and you see “LT,” you’ll have a rougher ride than you want. The three-digit number after the letter is the section width in millimeters. P225 means the tire is 225 millimeters wide from sidewall to sidewall. That’s roughly 8.8 inches. A 265 tire is about 10.4 inches wide. A 315 is about 12.4 inches – that’s for the wide tires you see on lifted trucks. The number tells you how much rubber meets the road. After the slash comes the aspect ratio. That’s the height of the sidewall as a percentage of the width. On a 225/60 tire, the sidewall height is 60% of 225 millimeters – about 135 millimeters. A lower number means a shorter sidewall. A 225/45 tire has a much shorter sidewall than a 225/60. That’s why low-profile tires look thin. They ride rougher, but they corner better. The letter after the aspect ratio tells you the construction type. R stands for radial. Almost all modern tires are radial. You might see a B for bias-ply on a trailer or a classic car, but that’s rare. The last two-digit number is the wheel diameter in inches. R17 means the tire fits a 17-inch wheel. That’s the rim size. A 20-inch tire won’t fit on a 17-inch wheel. A 17-inch tire won’t stay on a 20-inch wheel. Seems obvious, but I’ve seen people try. That’s the size. But the sidewall has more information. After the size, you’ll see a number like 94V or 115T. That’s the load index and speed rating. The Tire Load Index Calculator can decode that for you.

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Tire Load Index Calculator

Enter your tire size and load index — see maximum weight per tire and recommended load for your vehicle.

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The load index is a two- or three-digit number that tells you how much weight each tire can carry. A 94 load index is about 1,477 pounds. A 115 load index is about 2,679 pounds. A 121 is about 3,197 pounds. The number goes up, the capacity goes up. You need tires that meet or exceed your vehicle’s gross axle weight rating. That number is on the sticker inside your driver’s door jamb. The letter after the load index is the speed rating. V is good for 149 mph. T is 118 mph. H is 130 mph. Y is 186 mph. The Tire Speed Rating Guide has the full chart.

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Tire Speed Rating Guide

Enter your tire speed rating — see maximum safe speed and performance characteristics.

All data stays in your browser — we never see it.

Most drivers will never hit 118 mph. That’s fine. But speed rating also affects heat tolerance. A higher rating means the tire can handle higher temperatures. If you tow heavy loads or drive in hot climates, don’t downgrade your speed rating. Beyond the size, load index, and speed rating, the sidewall has other useful information. Look for “M+S” or the three-peak mountain snowflake symbol. M+S means mud and snow – an all-season tire. The three-peak mountain snowflake means the tire meets winter traction standards. If you live in Michigan, you want that symbol on your winter tires. Look for the DOT code. It starts with “DOT” followed by a series of letters and numbers. The last four digits tell you the week and year the tire was made. 4322 means the tire was made in the 43rd week of 2022. Tires older than six years should be replaced, even if they have tread left. Rubber hardens. Cracks form. Traction drops. I remember a guy named Frank. He bought a used Ford Explorer with tires that looked brand new. Good tread depth. No visible damage. He drove it for two years. Then one of the tires blew out on I-75. The tire was eleven years old. The DOT code showed 3213. Frank had no idea tires expire. He was lucky he didn’t crash. The Tire Size Comparator can help you see the difference between old and new tires.

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Tire Size Comparator

Compare two tire sizes side by side — see diameter, width, sidewall, and revs per mile.

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If you’re buying used tires, check the DOT code first. If the tires are more than four years old, discount the price heavily. If they’re more than six years old, walk away. Another mark on the sidewall: the treadwear rating. It’s a three-digit number on tires that meet UTQG standards – Uniform Tire Quality Grading. A treadwear rating of 300 means the tire is predicted to last three times as long as a tire rated 100. Higher numbers last longer but sometimes have less grip. It’s a trade-off. A summer performance tire might have a rating of 200. An all-season touring tire might have a rating of 600. You choose based on your priorities. Traction rating is also there: AA, A, B, or C. AA is the best wet braking performance. C is the worst. Most passenger tires are A. That’s fine. If you drive in rain a lot, look for AA. Temperature rating is A, B, or C. A is the best heat resistance. C is the worst. Don’t buy a tire with a C temperature rating unless you only drive around town at low speeds. A friend of mine, a guy named Rob, bought a set of used wheels for his Mustang. The tires had a treadwear rating of 500, which seemed great. He didn’t notice the temperature rating was C. On a hot summer day, driving aggressively on a back road, the tires overheated. The rear end stepped out. He spun into a field. No damage to the car, but his ego took a hit. He checked the sidewall after. C rating. He threw those tires away. The sidewall also tells you the maximum inflation pressure. Usually it’s around 44 psi for passenger tires, 80 psi for LT tires. That’s the maximum safe pressure, not the recommended pressure for your car. The recommended pressure is on the door jamb sticker. Use that one. One more thing: run-flat tires have a symbol. Usually it’s a little circle with a flat tire icon inside. Or a label like “RFT” or “SSR.” Run-flats can be driven for about 50 miles after losing pressure. They ride stiffer. They cost more. If your car came with run-flats, you can replace them with standard tires, but you won’t have a spare. Some cars don’t carry a spare. Know that going in. I spent about ten minutes with a coworker’s son last week. He had just bought a used WRX. The tires were mismatched – two different brands, two different tread patterns, two different load indexes. The fronts were 91V. The rears were 94W. He had no idea. The car drove fine, he said. I told him to check the sidewall. He did. His jaw dropped. He ordered a matching set that night. The Tire Speed Rating Guide helped him pick the right one. His car originally came with 93Y tires. He bought 93Y replacements. Reading a tire sidewall isn’t hard. It’s just unfamiliar. Once you know what the letters mean, you can spot problems in seconds. The wrong load index. The wrong speed rating. Old tires. Mismatched tires. All of it is right there, in plain text. You just have to look. Next time you’re at a tire shop, don’t just nod. Crouch down. Read the sidewall. Compare it to the door jamb sticker. If the numbers don’t match, ask why. The guy at the counter might look at you funny. That’s fine. It’s your money. It’s your safety. Don’t trust them to get it right.