Tire Sizing Explained: Reading the Sidewall and Choosing a Bigger Tire

Tires are the upgrade most owners make first and the one most often made without understanding what the size on the sidewall means. There are two systems in use. Passenger and light-truck tires use a metric format such as 285/70R17, and many off-road tires use a flotation format such as 33x12.50R17. Side-by-side and ATV tires use a variation of the second. Once you can translate between them, you can predict how a tire will fit, how it will affect your gearing and what it will do to your speedometer.

In the metric format, the first number is the section width in millimeters, measured across the widest part of the tire. The second is the aspect ratio: the sidewall height as a percentage of the width. The letter R means radial construction, and the final number is the wheel diameter in inches. So a 285/70R17 is a tire roughly 285 millimeters wide, with a sidewall that is 70 percent of that height, on a 17-inch wheel. The overall diameter is the wheel plus two sidewalls, which you can work out with a calculator.

The flotation format is easier to read because it states the answer up front. In 33x12.50R17 the first number is the overall diameter in inches, the second is the width in inches and the last is the wheel diameter. When someone says they are running 33s or 35s, this is the number they mean. Side-by-side tires use the same idea, typically in sizes such as 30x10R14, and the difference between a stock size and an upgrade of an inch or two matters as much on a RZR-XP platform, 2014 to 2026, as it does on a truck.

The reason overall diameter matters is that it changes everything downstream. A larger tire lifts the vehicle, which is the point, but it also lowers the effective gear ratio, so the engine turns more slowly for a given road speed. Acceleration suffers and, on an automatic, the transmission may hunt between gears. Your speedometer and odometer read low because each revolution covers more ground than the computer expects. On modern vehicles a dealer or a programmer can correct the reading; on older ones you live with it.

Width has its own consequences. A wider tire gives a bigger footprint in sand and mud, but it also weighs more, is harder to turn at low speed, and is more likely to rub the fender, the frame or the suspension at full lock and full compression. Every extra inch of width needs somewhere to go. Wheel offset, the distance between the mounting face and the centerline of the wheel, decides whether that extra width goes inward toward the suspension or outward past the fender.

Fit is a question of three things: diameter, width and offset together. A Wrangler JL, 2018 to 2026, will take a modest increase over stock on the factory suspension. Larger tires need a lift, and the parts listings on this site note which lifts clear which sizes without trimming. A Tacoma on the N300 platform, 2016 to 2026, or a 4Runner on the N280 platform, 2010 to 2026, has less room in the front wheel well than a Jeep, and rubbing at full lock is the usual result of guessing. Ask owners of the same platform and year what they run before ordering.

Beyond the size, the sidewall carries a load index and a speed rating as a code, and the tire's load range, which describes how many plies and how stiff the carcass is. These figures are printed on the tire and in its documentation, and you should read them there rather than in an article. The important idea is that a tire chosen for a heavy truck will ride stiffly on a light vehicle, and a light tire on a heavy truck may not carry the load safely, particularly when fully laden on a trip.

Tread pattern is the last decision. All-terrain tires have a closely spaced tread that is quiet on the road and works well on gravel and hard-packed dirt. Mud-terrain tires have large, widely spaced blocks that clear mud and grip rock but are noisy and wear faster on pavement. Between them are hybrid patterns that try to split the difference. Your riding decides. A truck that spends most of its life on the road and sees rock a few weekends a year is an all-terrain truck.

On a side-by-side the same logic applies with one addition: unsprung weight. A heavier tire and wheel combination on a light machine has a noticeable effect on suspension performance and on the drive belt, which has to spin more mass every time you accelerate. Owners of a Maverick X3 on the X3 platform, 2017 to 2026, who move to a much larger tire often find that the belt and the clutching, not the engine, become the limit. Plan the tire and the gearing together.

Whatever size you land on, follow the truck maintenance guidance and rotate the tires and inspect the sidewalls every 5,000 miles, because rock damage shows on the sidewall before it shows anywhere else. Check the pressure with the same gauge every time, and set it from the figure in your manual or the tire maker's guidance rather than from memory. A good tire, fitted properly and looked after, is the most valuable thing between you and the trail.