The short answer
Going from an 8 in wheel at +35 mm to a 9 in wheel at 0 mm: backspacing goes from 5.88 in to 5.00 in. The new wheel sits 1.88 in further out toward the fender and gives 0.88 in more room on the inside.
Worked example
| Current: 8 in, +35 mm | New: 9 in, 0 mm | |
|---|---|---|
| Backspacing | (8 + 1) ÷ 2 + 35 ÷ 25.4 = 5.88 in | (9 + 1) ÷ 2 + 0 = 5.00 in |
| Front spacing | 9 − 5.88 = 3.12 in | 10 − 5.00 = 5.00 in |
| Change | Inside: +0.88 in room | Outside: +1.88 in poke |
Offset is the distance from the wheel’s centerline to its mounting face: positive offset pulls the wheel in toward the suspension, negative pushes it out. A lower offset or a wider wheel moves the outer edge toward the fender. Measured backspacing (a straightedge across the back of the wheel) beats the formula, so you can enter it. This is geometry only: it cannot tell whether the wheel clears your brakes, suspension or fender.
Always confirm the wheel width range for the tire, clearance at full steering lock and suspension travel, and the load index and speed rating for your vehicle. See how to read a tire size, the load index chart and the speed rating chart.
FAQ
How do I calculate wheel backspacing?
Backspacing = (width + 1) ÷ 2 + offset in mm ÷ 25.4, which assumes about ½ in of flange on each side. A 9 in wheel at 0 mm offset has 5.0 in of backspacing.
What does positive offset do?
It moves the mounting face toward the outside of the wheel, so more of the wheel sits inboard toward the suspension. Negative offset pushes the wheel out toward the fender.
Does a wider wheel stick out more?
A wider wheel grows both ways. At the same offset it adds half of the extra width on each side of the centerline.
Will the new wheel fit?
This tool does not say. Check brake, suspension and fender clearance, steering lock and tire width.