What the calculator compares
- Wheel width: the rated width in inches, the 9 in a 17x9 wheel.
- Offset: the distance from the wheel centreline to the hub mounting face, in mm. Lower (or negative) offset moves the wheel out; higher offset moves it in.
- Tire (optional): a size code like 265/70R17, or just the section width in mm. Leave the new tire empty to keep the same tire. With a tire, the edges are whichever sticks out further, tire or wheel.
Every position is measured from the hub face, outboard positive. Poke is how much further out the outer edge sits. Inner edge toward suspension is how much closer the inner edge comes to the suspension; a negative figure means it moves away and you gain room there.
The formula
centreline = −offset (mm) ÷ 25.4
wheel outer edge = centreline + (width + 1) ÷ 2
wheel inner edge = centreline − (width + 1) ÷ 2
tire edges = centreline ± tire width ÷ 2
poke change = new outer edge − current outer edge
track change = 2 × (new centreline − current centreline)The 1 in added to the width is the usual 0.5 in allowance for each flange, the same convention as the backspacing calculator.
Worked example
From a 17x8 at +35 mm to a 17x9 at −12 mm, both with 265/70R17 tires (10.43 in wide). The centreline moves from 1.38 in inboard of the hub face to 0.47 in outboard, a shift of 1.85 in. The tire sticks out 1.85 in further, and its inner edge moves 1.85 in away from the suspension. Backspacing drops from 5.88 in to 4.53 in, and the track widens by 3.70 in across the axle.
Compare wheels only (leave the tire empty): the outer wheel edge moves out 2.35 in, because the new wheel is also 1 in wider and half of that goes to each side.
Same offset, wider wheel
Going from an 8 in to a 9 in wheel at the same +35 mm offset adds 0.5 in on each side: 0.5 in more poke and 0.5 in closer to the suspension. That inner half inch is often the one people forget.
What this does not tell you
The tire is drawn at its nominal section width. A real tire bulges a little more or less depending on the model and the rim width, and the tightest spots are often at full steering lock or with the suspension compressed. Use these numbers to narrow your choice, then check the real clearance on your vehicle.
Questions
How much further out will a lower offset wheel sit?
For the same width, every 25.4 mm less offset moves the wheel 1 in out. Going from +35 mm to +20 mm on an 8 in wheel moves it 0.59 in out, and 0.59 in away from the suspension.
Does a wider wheel with the same offset stick out more?
Yes, by half the extra width on each side. An 8 in to 9 in wheel at the same offset pokes out 0.5 in more and sits 0.5 in closer to the suspension.
What does going from +35 to −12 offset do?
The wheel centreline moves 47 mm, or 1.85 in, outward. With 8 in to 9 in wheels and the same tire, the tire pokes out 1.85 in more and the track widens by 3.70 in.
Will these wheels fit my vehicle?
The calculator cannot tell you that. It shows how the new setup moves compared with the one you have now; if your current setup clears, the change tells you how much margin you need. Measure or test fit to be sure.