Knowledge Base / Fitment
How to Measure Wheel Offset
The number that decides whether a wheel rubs the suspension or sits proud of the arch, measured in five minutes.
The number that decides whether a wheel rubs the suspension or sits proud of the arch, measured in five minutes.
Offset is the distance from the wheel's mounting face to the centre of the rim, measured in millimetres. Positive means the mounting face sits outboard of the centre, negative means it sits inboard. Get it wrong and the wheel either rubs the suspension or sticks out past the arch.
A straight edge long enough to span the rim, a tape measure, and a flat floor. A second pair of hands helps but is not required.
1. Lay the wheel face down on a flat surface, tyre off if you can. If the tyre stays on, the measurement gets fuzzy — the sidewall bulges and the straight edge rocks.
2. Measure the overall width from outer lip to inner lip. This is not the advertised width. A wheel sold as 9.5 inches usually measures about 10.5 inches across the outside of the lips, because the advertised figure is between the bead seats.
3. Measure backspacing: put the straight edge across the inner lip, drop the tape to the mounting face, and read the distance.
4. Do the arithmetic: offset = backspacing − (overall width ÷ 2), both in millimetres.
Overall width measures 267 mm. Backspacing measures 160 mm. Half the width is 133.5 mm. Offset is 160 − 133.5 = 26.5 mm, so the wheel is ET26.
If the same wheel measured 110 mm of backspacing, the offset would be 110 − 133.5 = −23.5 mm, a deep-dish fitment that pushes the wheel outward.
Using the advertised width instead of the measured width shifts the result by 12–13 mm, which is the difference between a fitment that clears the brake caliper and one that does not.
Measuring with the tyre fitted adds the sidewall to the straight edge, and the error goes straight into the number.
Measuring on a car, with the wheel bolted up, tells you nothing about offset. It only tells you where the wheel currently sits.
If you are ordering a set, you do not need to measure anything. Send the car's make, model, year and trim, the brake package if it is not standard, and a photo of the current wheel from the side. We check the PCD, centre bore, offset range and brake clearance against the drawing before the order is released.
If you already have a wheel on the bench and want to know what it is, the four steps above give you the answer in about five minutes.