Fitment is where most builds start and where most of them go wrong. A wheel and tire package has to clear the strut, the fender and the brake caliper, keep the speedometer honest, and put the tread where the suspension geometry expects it. Four numbers do almost all of that work: overall diameter, width, offset and bolt pattern.
Diameter is the one people underestimate. Change it and you change your effective final drive, your speedometer reading, your ABS calibration and your ride height all at once — which is why the tire calculator reports speedometer error alongside the size comparison. A 3% diameter increase is roughly 3% off your indicated speed and 3% taller gearing.
Offset (stamped ET on the wheel) decides where the wheel sits inside the arch. It is measured from the mounting face to the wheel's centreline, so a lower ET pushes the wheel outward. Two wheels of the same width and different offset can differ by an inch of clearance on the inner edge — the difference between clearing your coilover and grinding it.
Bolt pattern is the pass/fail gate before any of that matters. Patterns cluster close together — 5×112 and 5×114.3 are 2.3 mm apart and will feel like they fit until the studs load unevenly — so measure rather than eyeball.
FAQ
How much bigger can I go without a speedometer error?
Keep overall diameter within about 3% of stock and the speedometer error stays around 3%, which most people never notice. Beyond that the error grows linearly, and on many cars the ABS and traction control are calibrated to the stock rolling circumference too.
Is a lower offset always more aggressive?
Lower offset pushes the wheel outward, so yes, visually. But it also increases scrub radius and the load on wheel bearings, and it moves the tire toward the fender under compression. Width and offset have to be considered together — a wider wheel at the same offset grows in both directions from the mounting face.