What is wheel offset?
Offset is the distance in millimeters from the wheel's centerline to the surface that bolts to the hub. It is stamped on the wheel as ET (for example ET35).
- Positive offset: the mounting face is toward the outside of the wheel, so the wheel sits further in. Most modern cars use positive offset.
- Zero offset: the mounting face is exactly on the centerline.
- Negative offset: the mounting face is toward the inside, so the wheel pokes out. Common on lifted trucks.
Backspacing is the distance from the mounting face to the inner edge of the wheel, in inches. It tells you how close the wheel comes to the suspension and brakes.
Wheel offset formulas
Backspacing = overall width ÷ 2 + offset (mm) ÷ 25.4
Front spacing = overall width − backspacing
Poke change = new front spacing − old front spacing
Worked example
Going from an 8 inch ET35 wheel to a 9 inch ET20 wheel: the old wheel has 5.88 in of backspacing and 3.12 in of front spacing; the new one has 5.79 in and 4.21 in. The new wheel sits about 1.09 in (28 mm) further out at the fender, and has 0.09 in (2 mm) more room on the inside. That much extra poke usually needs a check for fender clearance.
Rules of thumb
- Lower offset = wheel moves out. Every 5 mm less offset moves the wheel about 0.2 in toward the fender.
- Wider wheel at the same offset grows equally in and out: half the extra width each side.
- Check both sides: the outer lip against the fender and the inner barrel against struts, control arms and brake calipers.
- Changing tire size too? Check diameter with the tire size calculator.
Frequently asked questions
What does a lower wheel offset do?
A lower offset moves the mounting face toward the inside of the wheel, so the wheel sits further out toward the fender. A higher offset tucks the wheel in toward the suspension.
How do I convert offset to backspacing?
Add 1 inch to the rim width to get the overall width, divide by 2, then add the offset converted to inches (mm divided by 25.4). An 8 inch ET35 wheel has about 5.88 inches of backspacing.
How much offset change is safe?
Changes of a few millimeters rarely matter. Beyond about 10 mm, or with wider wheels, check fender and suspension clearance and consider rolling the fenders. Staying close to the factory offset also keeps steering feel and wheel bearing loads similar.
What does ET mean on a wheel?
ET comes from the German word Einpresstiefe and is the wheel's offset in millimeters. ET35 means a positive 35 mm offset.
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Last updated October 2026.