What spacers do (and don't do)
A wheel spacer pushes your wheel outward — same effect as a lower offset. It's the cheapest way to fix wheels that sit too tucked, or to clear big brakes. What it doesn't do: change your bolt pattern, fix the wrong center bore, or make an unsafe setup safe. Measure first, buy once.
The 5-minute measuring method
To measure for wheel spacers, park on flat ground with the suspension loaded and hold a straight edge vertically against the fender lip. Measure the gap from the straight edge to the outer face of the tire at all four corners, then subtract 5 mm for suspension travel. That number is your safe spacer thickness — use hub-centric spacers only.
Park on flat ground, wheels straight. Hold a straight edge (a level, a yardstick) vertically against the fender lip, extending down past the wheel. Measure the gap between the straight edge and the outer face of the tire — that's your available poke room. Do it at all four corners (they differ), and do it with the suspension loaded (car on the ground, not on a jack). Subtract 5mm for safety — suspension compresses over bumps, and "flush at rest" becomes "rubbing at speed."
How thick is safe?
3–5mm: subtle, usually fine on stock studs. 10–15mm: the sweet spot for most cars — but check stud engagement (below). 20–25mm: aggressive; use bolt-on style spacers with their own studs. Beyond 25mm: you're changing suspension geometry meaningfully — get the right offset wheels instead. Spacers are a correction tool, not a substitute for correct wheels.
Hub-centric or nothing
Your spacer must be hub-centric: it centers on your car's hub and provides a new hub lip for the wheel to center on. Universal flat spacers leave the wheel hanging on the lugs — vibration, uneven load, bad news. Match the spacer's center bore to your car exactly, and make sure it has the extended hub lip.
The stud rule (this is the safety-critical part)
Your lug nuts/studs need full thread engagement — the rule is at least as many turns as the stud diameter (e.g. a 12mm stud needs ~12 full turns, roughly 7+ turns minimum in practice). Slip-on spacers eat into that engagement. If a 15mm spacer leaves you with 4 turns, you need extended studs or bolt-on spacers. This isn't a style question — a wheel coming off at speed is a catastrophe. When in doubt, go bolt-on.
Torque and maintenance
Torque spacer bolts and lugs to your car's spec with a torque wrench, in a star pattern. Re-torque after 50–100 miles — spacers settle and this step catches it. Check them at every tire rotation. Quality spacers are machined aluminum, anodized, with proper corrosion resistance — not the cheapest cast set with no specs listed.