How to read a tire size, and what plus-sizing really changes

Every tire decision starts with a string of numbers most people glance at and forget: 285/70R17. That code is not marketing. It is a complete set of dimensions, and once you know how to read it you can predict a tire's diameter, sidewall height, revolutions per mile, and what it will do to your speedometer and gearing before you spend a dollar. This guide walks through the code number by number, then covers the part that actually matters: what changes, and what does not, when you move to a bigger tire.

Reading the size, number by number

Take a common truck size, 285/70R17, and split it into its four pieces:

• 285 is the section width in millimeters, measured sidewall to sidewall on the rim width the manufacturer used for the measurement. 285 mm is about 11.2 inches of width.
• 70 is the aspect ratio: the sidewall height expressed as a percentage of the width. The sidewall of a 285/70 is 70 percent of 285 mm, which is 199.5 mm, or about 7.85 inches. This is the number people most often ignore, and it is the one that sets the tire's overall diameter together with the rim size.
• R simply means radial construction, which describes how the internal cords are laid. Nearly every passenger and light-truck tire sold today is a radial. (A "D" in this spot would mean bias-ply diagonal construction, mostly seen on trailer and vintage applications.)
• 17 is the wheel diameter in inches. This is the one number that must match your wheels exactly.

From those four numbers you can compute the overall diameter, which is the dimension that drives almost everything else:

Diameter = wheel diameter + (2 × width × aspect ratio), with the sidewall converted from millimeters to inches. For the 285/70R17: sidewall = 285 × 0.70 ÷ 25.4 = 7.85 inches, and diameter = 17 + (2 × 7.85) = 32.7 inches.

Two more markings are worth knowing. The service description follows the size, something like "116T": 116 is a load index (a coded maximum weight per tire, 2,756 lb for 116) and T is a speed rating (118 mph sustained). Trucks and SUVs that tow often carry an LT prefix instead, for light truck, with a load range letter (C, D, E) describing casing strength. A P-metric tire and an LT tire in the identical size are not interchangeable in capability: the LT is built for higher inflation pressure and load, rides firmer, and weighs more. Note also that when a P-metric tire is fitted to a light truck, the tire industry derates its load capacity by a factor of 1.1, which is one reason many trucks ship with LT rubber from the factory.

Finally, find the four digits in an oval on the sidewall, the end of the DOT code. 2423 means the tire was built in the 24th week of 2023. Age matters independently of tread depth: rubber hardens and cracks with time and heat, and most manufacturers suggest professional inspection after five years in service and replacement by ten years from manufacture regardless of how the tread looks.

The sizes side by side

Here are four sizes you will see on trucks every day, worked out with the formula above:

Revolutions per mile is just 63,360 inches (one mile) divided by the tire's circumference. It is the number your speedometer, odometer, transmission shift schedule, and stability systems are all quietly counting. Inch-format sizes like the 33×12.50R15 simply state the diameter and width directly, which makes the math trivial: sidewall is (33 − 15) ÷ 2 = 9 inches.

What actually changes when you plus-size

"Plus sizing" originally meant moving to a larger wheel diameter while shortening the sidewall to keep the overall diameter the same: a 245/70R17 (30.5 inches tall, 6.8-inch sidewall) becomes a 245/65R18 at the same 30.5-inch diameter with a 6.3-inch sidewall, or a 245/60R19, again at 30.5 inches. Overall diameter is preserved, so gearing and the speedometer are untouched; what changes is steering response (a shorter sidewall flexes less), appearance, and how much protection the tire gives the wheel against potholes.

What most truck owners actually do is different: they keep the wheel and fit a taller tire. Going from a 265/70R17 to a 285/70R17 adds 1.1 inches of diameter (+3.5 percent) and 20 mm of width. Here is what that one change does:

• Speedometer and odometer read low. The truck counts revolutions, and each revolution now covers 3.5 percent more ground. When the dash says 65 mph, you are really traveling about 67 mph. Tickets and warranty mileage both follow the corrected number, not the displayed one.
• Effective gearing gets taller. A larger tire is a longer lever against the axle. Your 3.73 axle behaves like a 3.60: multiply the ratio by (old diameter ÷ new diameter). Acceleration softens slightly, and the transmission may hold gears longer or downshift more on grades.
• Clearance is spent in three directions at once. Half the diameter gain (0.55 inch here) goes up into the fender, half goes down as ground clearance gained. The extra 20 mm of width spends clearance at the inner sidewall against suspension and steering parts, which is why wheel offset matters as much as tire size.
• Braking and feel change modestly. The taller tire increases the leverage the brakes must overcome and usually adds unsprung weight. Modern trucks absorb a one-step change without drama; a two-step jump is where pedal feel and stopping distances start to be worth measuring.

A widely used sanity limit is to stay within about 3 percent of the stock overall diameter if you want to avoid reprogramming the speedometer and rethinking gears. Beyond that, plan on correcting both, and on a four-wheel-drive truck remember the spare: a mismatched spare diameter makes the drivetrain bind on dry pavement, so the spare should match the set.

Frequently asked questions

Will a bigger tire make my speedometer read fast or slow?

Slow. A taller tire covers more distance per revolution than the truck was calibrated for, so your actual speed is higher than displayed. A shorter tire does the opposite. The error is a fixed percentage at every speed, which is why it is small in town and meaningful on the highway.

How much diameter change is too much?

Up to roughly 3 percent, most trucks need nothing but awareness. Between 3 and 5 percent, correct the speedometer (a programmer or dealer flash) and expect softer acceleration. Beyond that, gearing changes and clearance work are usually part of the job, not options.

Can I tell whether a tire will rub from the size alone?

No. Rubbing depends on the combination of tire diameter, tire width, wheel width, and wheel offset, plus the specific truck's fender and suspension geometry. Size math tells you how much clearance you are spending; the offset guide below explains where the wheel positions that tire.

Does a wider tire always grip better?

A wider tire makes a wider, shorter contact patch of roughly the same area (the area is set mostly by load and inflation pressure). Lateral grip on dry pavement usually improves, but a wide tire hydroplanes sooner in standing water and follows ruts more. Width is a trade, not a free upgrade.