How to read tire sidewall numbers to know tire size, rim size and more
There's a lot of info there when you want to buy the best new tires for the best price.
- 5G Technician, ETA International
You can read automotive tires like a book if you know the language, and doing so will help you buy the best ones and get the most miles out of them. Our video above takes you through the entire secret world hiding in plain sight on the side of your tire, but here are a few highlights.
Width, height and size
Perhaps the most commonly discussed piece of data about a tire is a number in the format "XXX/XXRXX," which describes a tire's size and shape in a somewhat arcane way. Using 235/55R18 as an example, 235 is the tread width in millimeters. The higher that first number, the wider the tire. The 55 is the height of the tire expressed as a percentage of the width we just saw. The lower this number, the shorter and more aggressively the tire sits on the wheel. The R means the tire is of radial construction (they all are) and the 18 is the diameter of wheel the tire fits, expressed in inches. This string is a real hodgepodge of numbers but it carries the essence of a tire's applicability.
Your tire's birthday
In smaller type, typically at the end of a line that begins with the letters DOT, you'll find a four-digit sequence like 3219. This reveals your tire's manufacturing date, expressed as the week of the year followed by the last two digits of that year. In this example "3219" means the tire was made in the 32nd week of 2019. This is important information because tires can run out of life long before they run out of tread: Heat, UV, ozone and the quality of the tire's materials can end a tire's safe usage life long before it fails the Lincoln penny tread test. The tricky part is knowing how old is too old.
This tire was made in the 14th week of 2017. The numbers before 1417 are manufacturing codes you or your tire installer should register to make sure you receive recall notices that apply to your rubber.
The largest tire chain in the US, Discount Tire/America's Tire, has a policy that suggests you replace tires after six years, which is a commonly mentioned rule of thumb though many consumers see it as a ploy to sell them new tires. The company won't even service tires that are over 10 years old, other than to take them off your car, but that isn't as definitive as it sounds. "There is no such thing as a 'freshness date' for tires," says Richard Sherman, tire and accident reconstruction expert at Robson Forensic. "I've seen tires fail in one year or last about 20 years," he says, adding that you should look for sidewall cracking or unusual tread appearance as signals that your tires could be failing in ways that aren't revealed by simple tread depth.
If there is anything like a universal truth about tire age it would be that you should replace any tire with a three-digit date code: The industry hasn't used those codes since 1999.
Under pressure
We've all heard the exhortations to check and maintain our tire pressure for the best fuel economy, handling and safety. All of that is true, but when you grudgingly stoop down at the gas station with that nasty tire inflator, all you see on your tires is a "MAX PRESSURE" number. That is not the proper inflation pressure, merely the highest at which the tire won't disintegrate or come off the wheel. The pressure you should inflate your tire to is located in one of a few places according to federal rules: On a door edge, door post or glove box door or inside the trunk, or on the fuel-filler door and in the owner's manual. That doesn't narrow it down much, but once you know the number it's easy to remember. Just don't inflate to the MAX PRESSURE on the side of the tire.