OEM vs. Aftermarket & Fitment Specs.
Learn the difference between OEM and aftermarket, and master the anatomy of a wheel.
The Two Classes of Wheels
Before diving into complex measurements, it's important to understand where your wheels come from and what they are built for.
OEM Wheels
Original Equipment Manufacturer. These are the wheels that were originally designed, engineered, and installed on the vehicle by the car manufacturer (e.g., BMW, Audi, Porsche) straight from the factory.
- Guaranteed exact fitment for the specific vehicle without hub rings or spacers.
- Rigorously safety tested for structural integrity and impact resistance.
- Very limited styles, widths, and aggressive offset options.
- Generally heavier (cast aluminum) and significantly more expensive when bought new from a dealership.
Aftermarket Wheels
Wheels produced by independent third-party companies (e.g., BBS, HRE, Vossen, Enkei) designed to replace or upgrade factory wheels. They cater to specific aesthetic and performance goals.
- Unlimited variety of designs, colors, finishes, and custom concavity.
- Ability to dial in aggressive, flush fitments (wider widths, lower offsets).
- High-end models (forged/flow-formed) are significantly lighter and stronger than OEM.
- May require centering rings (hub rings) or tuner lug nuts/bolts to fit correctly.
The Anatomy of a Wheel
Understanding these exact measurements ensures your new wheels will clear your brakes, suspension, and fenders safely.
Diameter e.g., 19"
The total distance across the face of the wheel in inches. When you buy tires, the last number (e.g., 255/35R19) must perfectly match the wheel diameter.
Width e.g., 9J
The width of the wheel measured in inches from the inside of the outer lip to the inside of the inner lip. This determines how wide of a tire you can safely mount.
Offset (ET) e.g., ET +35
The distance in millimeters from the wheel's centerline to the hub mounting pad. A higher positive offset pushes the wheel deep inside the fender. A lower (or negative) offset pushes the wheel outward for a wider stance.
PCD (Bolt Pattern) e.g., 5x114.3
Pitch Circle Diameter. The first number is the amount of lug holes (e.g., 5). The second number is the diameter of the imaginary circle drawn through the center of those holes in millimeters.
Center Bore (CB) e.g., 66.6mm
The size of the machined hole in the back of the wheel. It must match the size of the hub on your car to ensure the wheel is perfectly centered. If the wheel's CB is larger, you must use a hub-centric ring.
Structure Forged / Cast
How the wheel is manufactured. Cast wheels are poured liquid aluminum (heavy, cheap). Flow-Formed stretches the barrel for strength. Forged wheels are milled from a solid block of aluminum (lightest, strongest, most expensive).
Need help?
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