What Does 255/70R18 Mean? | Tire Code Decoded

255/70R18 describes a tire’s width, sidewall height, construction, and wheel size: 255 mm wide with a 70% aspect ratio, radial build, for 18-inch rims.

If you’ve stared at the sidewall of a truck or SUV tire and wondered what the string of numbers actually means, you’re not alone. The code 255/70R18 is less cryptic than it looks — each part names one physical dimension of the tire. Here’s the breakdown, plus the specs that matter when you’re shopping for a replacement.

The Meaning Of Each Number And Letter

Every tire code follows the same format, and 255/70R18 breaks down into four distinct pieces of information. Once you know what each one stands for, you can read any metric tire size on sight.

• 255 — the section width in millimeters, measured from the widest point of the sidewall to the widest point of the other sidewall, not the tread.
• 70 — the aspect ratio, meaning the sidewall height is 70% of the section width. For a 255 mm tire, that works out to roughly 178.5 mm, or about 7 inches.
• R — radial construction, the current standard for nearly all passenger and light-truck tires, where the internal cords run perpendicular to the direction of travel.
• 18 — the rim diameter in inches. This tire mounts only on an 18-inch wheel, and the number has nothing to do with wheel width.

The most common mistake people make is reading “18” as the width of the wheel. It’s the diameter, and it determines which rims the tire will physically fit. Mounting a 255/70R18 on anything other than an 18-inch wheel isn’t just a mismatch — it’s a safety hazard.

What The Specs Mean For Trucks And SUVs

This size is a common fitment on trucks and SUVs, and the measurements show why. With a 32.1-inch overall diameter and a tread width of about 10 inches, the 255/70R18 delivers a tall, stable footprint that suits heavier vehicles.

For practical purposes, that tells you how the tire’s size affects your speedometer reading and gearing. If you swap to a significantly larger or smaller overall diameter, your speedometer will read wrong until the vehicle is recalibrated.

That overall diameter is the number to watch when comparing tires. Two tires can share the same 255/70R18 code and still differ in the details that matter at purchase time.

Load Rating, Speed Rating, And Treadwear Vary By Model

Here’s the part that surprises most buyers: the tire size only defines the physical dimensions. Two 255/70R18 tires can have completely different load capacities, speed ratings, and treadwear grades.

Load range SL (Standard Load) is common across all-weather and winter versions of this size.

What this means in practice:

• Load index — the maximum weight each tire can support at proper inflation. Higher number = higher capacity.
• Speed rating — the maximum sustained speed the tire is certified for. H (130 mph) and S (112 mph) are both common on this size.
• UTQG — treadwear, traction, and temperature grades. The treadwear number is comparative, not a mileage guarantee.