What Does 255/60R18 Mean? Tire Size Explained

255/60R18 means the tire is 255 millimeters wide, with a sidewall height equal to 60% of that width, radial construction, and a fit for an 18-inch wheel.

For the full breakdown, see our best 255/60R18 Tires guide.

If you’ve stared at the sidewall of a tire and wondered what the string of numbers and letters actually means, you’re in the right place. The code 255/60R18 isn’t arbitrary—each part carries a specific measurement. Understanding it tells you whether a tire fits your vehicle, how it handles, and what its physical dimensions are. One wrong interpretation can lead you to buy a tire that doesn’t fit your wheels at all.

The 255/60R18 Tire Size Code Breakdown

The code breaks down into four distinct pieces of information: width, aspect ratio, construction type, and wheel diameter. Here’s what each one means:

• 255 — Section width in millimeters (the widest point from sidewall to sidewall).
• 60 — Aspect ratio: the sidewall height is 60% of the width. That works out to 153 mm, or about 6 inches.
• R — Radial construction, meaning the internal ply cords run radially across the tire.
• 18 — Wheel diameter in inches. This tire only fits an 18-inch rim.

The aspect ratio is the part most people misread. A 60-series tire has a noticeably shorter sidewall than a 75-series tire, which changes handling and ride comfort. A lower aspect ratio generally means sharper steering response but a firmer ride.

What Are The Physical Dimensions Of A 255/60R18 Tire?

When mounted and inflated, a 255/60R18 tire measures roughly 30 inches in overall diameter and about 10 inches in tread width.

Here’s the full set of calculated figures for this common SUV and crossover size:

These numbers matter if you’re replacing tires and want to keep your speedometer accurate, or if you’re considering going up or down a size.

Why Getting The Load Index And Speed Rating Right Matters

The tire size alone doesn’t tell you everything. As Pirelli’s 255/60R18 108H listing shows, the same physical size can come with different load and speed capabilities. In that code, 108 is the load index (how much weight the tire can support) and H is the speed rating (the maximum speed the tire is certified for). A tire that fits your wheel might still be wrong for your vehicle if the load index is too low.

Common Mistakes When Reading 255/60R18

Three mix-ups trip up most drivers. First, assuming 255 is the wheel diameter—it’s not; it’s the tread width in millimeters. Second, reading 60 as inches instead of a percentage—the sidewall is 60% of 255 mm, not 60 inches. Third, thinking 18 describes the tire’s own size—it actually describes the wheel it mounts on.