How to read tire sidewall numbers to know tire size, rim size and more

You can read automotive tires like a book if you know the language, and doing so will ensure you buy the best ones and get the most miles out of them. Here are a few highlights from our video above that takes you through the entire secret world hiding in plain sight on the side of your tire. 

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 aggressive 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 hodge-podge of numbers but it carries the essence of a tire’s applicability.

Freshness date

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 the tire was made in the 32nd week of 2019. This is important information because tires generally last about six or seven years regardless of mileage due to the effects of heat, UV and ozone. If you find a three-digit date code, get new tires: Such codes are only found on tires made before the year 2000, making them dangerous antiques.

Tire sidewall OGI

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.

Emme Hall/Roadshow

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, 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. 


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