Wheel spacer do exactly what their name implies: they move the wheel out from its hub, widening the overall track width, either for cosmetic or functional reasons. They range from low-priced, universal, stamped spacers to model-specific, high-dollar, machined billet parts.
Spacers are usually made of aluminum and come in widths from 3mm to more than 2 inches. They mount behind the wheels using the wheel bolts or studs, and push the wheels out toward the fender.
Cosmetically, moving the mount point of a wheel out toward the fender lip can bring the wheel face flush with the fender, making the wheel appear wider and filling in the gap created when a wheel sits too far in.
The larger calipers common on brake upgrades can rub the backs of wheels, and installing spacers can bring back the necessary gap, eliminating the need for modification of expensive wheels or calipers.
Installing aftermarket wheels with larger-than-factory offsets can mean rubbing on the inner fender, and installing a spacer can bring the wheel back into spec. For instance, if the wheels you want only come in a 48 mm offset, but your stock wheels are 38 mm, using a 10 mm spacer would make the final offset 38 mm (48 mm - 10 mm spacer = 38 mm final offset) and, all else being equal, usable on your car.
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