What the inputs mean
- Wheel width: the rated width in inches, the 8 in a 17x8 wheel.
- Wheel offset: the distance from the wheel centreline to its mounting face, in mm. It is often stamped as ET35 or +35.
- Spacer thickness: the full thickness between the hub face and the wheel. Spacers are sold in inches and in millimetres; 1 in is 25.4 mm.
- Tire (optional): with a size, the outer and inner edges are whichever sticks out further, tire or wheel.
The formula
A spacer moves the wheel's mounting face outward by its thickness. The wheel then behaves exactly like a wheel with less offset:
effective offset (mm) = wheel offset (mm) − spacer thickness (mm)
new backspacing = old backspacing − spacer thickness
poke change = spacer thickness
track change = 2 × spacer thickness
spacer for a target = wheel offset − target offset
backspacing = width ÷ 2 + 0.5 in + offset ÷ 25.4Backspacing uses the usual 0.5 in flange allowance, the same as the backspacing calculator.
Worked example
A 17x8 wheel at +35 mm with a 1 in (25.4 mm) spacer on each side. The effective offset is 35 − 25.4 = +9.6 mm. Backspacing measured from the hub face drops from 5.88 in to 4.88 in, and frontspacing grows from 3.12 in to 4.12 in. With 265/70R17 tires, the tire sits 1.00 in further out, its inner edge gains 1.00 in of room from the suspension, and the track widens by 2.00 in.
Going the other way: to make a 7.5 in +45 mm wheel behave like +20 mm, you need 45 − 20 = 25 mm of spacer.
What a spacer changes beyond the numbers
Moving the wheel out moves the tire's contact patch further from the steering axis and the wheel bearing. That changes the scrub radius and the leverage on the bearing, so follow the spacer maker's instructions for your vehicle. Hub-centric spacers centre the wheel on a hub ring; lug-centric ones rely on the lug nuts. Read the spacer maker's instructions for which yours is.
What this does not tell you
The numbers show how far the wheel moves. They cannot tell you whether it clears your fender, or whether the studs still hold enough thread. Measure your clearance and check engagement with the spacer maker.
Questions
How does a wheel spacer change offset?
Subtract the spacer thickness from the wheel offset. A +35 mm wheel with a 25.4 mm (1 in) spacer behaves like a +9.6 mm wheel.
How much further out does a 20 mm spacer push the wheel?
20 mm, or 0.79 in, on that side. With spacers on both sides of an axle, the track widens by 40 mm, or 1.57 in.
What spacer do I need to go from +45 to +20 offset?
45 − 20 = 25 mm. On a 7.5 in wheel that takes the backspacing from 6.02 in to 5.04 in.
How much stud thread do I need with a spacer?
The calculator does not give a figure. It depends on the stud, the nut and the spacer design, so use the spacer maker's and the stud maker's instructions for thread engagement and torque.