What offset (ET) means
Offset is the distance from the wheel's centerline to the face that bolts to the hub, in millimeters. Positive offset (ET45) puts the mounting face toward the outside, so the wheel sits further in. Lower or negative offset pushes the wheel out toward the fender.
The math
Half the rim width sits each side of the centerline. So the outer lip sits width × 25.4 ÷ 2 − offset mm outboard of the hub face, and the inner lip sits width × 25.4 ÷ 2 + offset mm inboard. A spacer works like lowering the offset by its thickness.
Example: 15x6 ET40 → 15x7 ET35
The extra inch of width adds 12.7 mm on each side. Dropping the offset 5 mm moves the whole wheel 5 mm out. The result is 17.7 mm more poke and 7.7 mm closer to the strut. That's a common "just fits" vs "rubs on bumps" difference, depending on the car, tire and ride height.
Before you buy
- Measure your current clearance to the strut and fender lip with the car on the ground, steering straight and at full lock.
- Remember ride height and camber: lowering and negative camber tuck the top of the tire in, which gives more fender room and less strut room.
- Check brake caliper clearance separately. It depends on spoke design, not just offset.