A wheel spec like 20x9 +18 8x170 125.1mm looks like a random string of numbers until you know what each segment means. It isn't random — every wheel is described the same way, in the same order, and once you can read one you can read them all.
Breaking down 20x9 +18 8x170 125.1mm
| Segment | Meaning | In this example |
|---|---|---|
| 20 | Diameter, in inches | 20-inch wheel |
| x9 | Width, in inches | 9 inches wide |
| +18 | Offset, in millimeters | 18mm positive offset |
| 8x170 | Bolt pattern — 8 lugs on a 170mm circle | Ford Super Duty pattern |
| 125.1mm | Center bore diameter, in millimeters | 125.1mm bore |
Some specs also include a load rating in pounds or kilograms, which matters most for dually and heavy-duty applications where the wheel needs to be rated for the vehicle's axle weight. Not every listing shows all five numbers, but the diameter, width, offset, and bolt pattern are the ones you need to check every time.
Diameter
Diameter is measured in inches, bead seat to bead seat — the same number a matching tire size references (for example, a 33x12.50R20 tire is built for a 20-inch wheel). Common truck and SUV diameters run from 17 inches on more off-road-oriented setups up through 22 and 24 inches on street-focused builds. Bigger diameters generally mean a shorter tire sidewall for the same overall tire height, which trades ride compliance for a more aggressive look and, often, better steering response.
Width
Width is also measured in inches, and this is the one buyers most often misjudge: it's measured bead seat to bead seat — the inner surfaces the tire actually seals against — not the outer lip-to-lip distance across the whole wheel face. A wheel with a deep lip design can look much wider than its stated width because the lip extends beyond the bead seats without adding mounting width for the tire.
Width matters because every tire size has a manufacturer-approved rim width range. Mount a tire far outside that range and the sidewall either stretches thin (too narrow a wheel) or bulges past its designed profile (too wide a wheel), in both cases changing handling and wear characteristics from what the tire was designed for.
Offset
Offset is the distance, in millimeters, between the wheel's true centerline and its hub-mounting surface. It determines how far the wheel sits in or out of the fender opening at a given width. We cover this in full, with worked examples, in What Is Wheel Offset? — it's worth reading before comparing two wheels of the same diameter and width, since offset (not diameter) is usually what decides whether a wheel will rub.
Bolt pattern
Written as lug count x circle diameter — 8x170 means 8 lugs arranged on a circle 170mm across. This is a hard requirement, not a preference: the wheel physically will not mount to a hub with a different bolt pattern. Common truck patterns include 6x135 and 6x139.7 on half-ton trucks and full-size SUVs, and 8x170 or 8x180 on Super Duty and 2500/3500-class trucks. See What Is a Bolt Pattern? for the full rundown by platform.
Center bore
The center bore is the diameter of the hole in the wheel's face that centers it on the vehicle's hub, in millimeters. A wheel's bore has to be equal to or larger than the vehicle's hub — never smaller, or it simply won't slide onto the hub. When a wheel's bore is larger than the hub (common on aftermarket wheels designed to fit multiple platforms), hub-centric rings fill that gap so the wheel still centers correctly rather than relying on the lug studs alone to locate it.
Load rating
Where listed, load rating tells you the maximum weight the wheel is rated to carry, usually per wheel. This is most relevant on dually and heavy-duty pickup applications where axle-rated capacity matters for towing and payload, and less of a factor on lighter SUV and half-ton builds where most wheels comfortably clear the vehicle's rated capacity.
Common truck and SUV wheel sizes
| Size | Typical use |
|---|---|
| 17x9 | Off-road-oriented builds running 33-35 inch tires with more sidewall |
| 18x9 | Balanced daily-driver size, wide tire selection |
| 20x9 | Common factory-adjacent street size |
| 20x10 | Slightly wider stance, still tire-friendly |
| 20x12 | Deep-lip, negative-offset stance builds |
| 22x12 | Larger-diameter stance builds, shorter sidewall |
What this means when you pair it with a tire
Diameter has to match between wheel and tire exactly (a 20-inch wheel needs a 20-inch tire). Width has to fall inside the tire's approved rim width range. Neither offset nor bolt pattern affects the tire itself, but both affect whether the wheel-and-tire combination clears the vehicle once mounted. That's why a "correct" wheel size on paper can still rub or look wrong on a specific truck — the diameter and width might be right for the tire, while the offset is wrong for the fender.
Shopping by size
Once you know the diameter and width you want, browse by size — for example 17x9, 18x9, 20x9, or 20x10 — or start from the full wheel size hub to compare every available diameter and width combination. For a deeper look at how offset and backspacing decide fitment beyond just diameter and width, see Backspacing vs Offset, or browse the complete wheel catalog to compare finishes and styles within any size.