LEARN FITMENT
Wheel fitment has its own vocabulary — offset, backspacing, PCD, poke, stretch — and getting one term wrong gets expensive fast. These guides explain each concept from scratch, in plain language, with the math shown. Pair them with the calculator to see every concept live on your own car.
Positive, negative, zero — what ET actually measures, and why 10mm of offset changes everything about how your wheels sit.
The muscle-car world talks backspacing, the import world talks ET. Here is how the two measurements relate and the formula to convert.
No stamped ET on the wheel? Measure it yourself in five minutes with tools you already own.
4x100, 5x114.3, 5x120 — how to read a bolt pattern, measure one, and know which cars you can swap wheels between.
The two words that describe every fitment on the road — and the millimeter math that decides which one your car does.
A 195 on a 9-inch rim looks wild and rides on a knife edge. Where the line sits between fitment tool and liability.
Wider in the back looks right on RWD metal — but diameter matching, rotation, and AWD electronics all have opinions.
A spacer is just negative offset you bolt on later. The safety questions are stud engagement and hub centering — both solvable.
Width in millimeters, sidewall as a percentage, diameter in inches — three unit systems in one code, decoded once and for all.
The suspension design behind your hub decides how much wheel fits before anything touches. Know which game you are playing.