Where the tire actually ends up
A wheel offset calculation tells you where the rim sits. It does not tell you where the tire sits, and the tire is what actually contacts things. This adds the tire's bulge — where it is wider than the wheel — on top of the wheel's own position.
Check what these edge positions leave against your fender and strut — fender clearance and suspension clearance both take a measured baseline.
Getting a number you can act on
- 01Enter wheel width and offset
The same figures the wheel offset calculator uses — width in inches, offset in millimetres.
- 02Enter the tire's section width
In millimetres, from the tire's size designation. A tire wider than the wheel bulges beyond the rim edges; a tire narrower than the wheel does not.
- 03Read both edge positions from the hub mounting face
These are the figures that matter for clearance — not the wheel's position alone, but where the tire's rubber actually ends up.
- 04Feed these figures into the clearance calculators
The outer edge figure is what the fender clearance calculator needs; the inner edge figure is what the suspension clearance calculator needs.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
bulge = max(0, (tire width − wheel width) ÷ 2)Split evenly across both sides when the tire is wider than the wheel it's mounted on — zero when the wheel is as wide or wider than the tire.
edge = wheel edge position (from offset/backspacing) + bulgeStarts from the same backspacing calculation the wheel offset and backspacing calculators use, then adds the tire's own contribution.
Why a wheel calculation alone understates the real fitment
This is the gap the wheel offset calculator's own content flags explicitly but does not close: a tire mounted on a wheel narrower than its section width bulges outward on both sides, and that bulge is exactly what a wheel-only calculation misses.
Fit a 285-section tire on a 9-inch wheel and the tire measures roughly 11.2 inches at its widest — about an inch of rubber sitting proud of the wheel's edges on each side. A fitment check that stops at the wheel's position alone will say a setup clears when the tire itself does not.
This tool closes that gap by combining both calculations into one figure — the actual position of the rubber that will contact the fender and the strut, not just the metal that sits inside it.
Wheel Fitment Calculator FAQ
Why isn't the wheel offset calculator enough on its own?+
It calculates where the rim sits, but the tire mounted on it can extend beyond the rim's edges if it's wider than the wheel. This tool adds that tire bulge to give the actual edge position.
How much does a tire bulge beyond the wheel?+
Half the difference between tire section width and wheel width, on each side. A 285mm tire on a 9-inch (228.6mm) wheel bulges about 1.1 inches per side.
What if the wheel is wider than the tire?+
There's no bulge in that case — the tire sits within the wheel's own width, so the fitment matches the plain wheel offset calculation.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01The Tire and Rim Association — standards filing (NHTSA docket)TRA has been the US standardising body for tire and rim interchangeability since 1903; this filing sets out dimensional practice.↗02TireGrades — minimum and maximum tire size for a given rim widthSummarises the approved rim-width range published for each section width by TRA and ETRTO.↗The wheel is only half the fitment. This calculates where the rim sits. The tire mounted on it reaches further out and further in than the rim does, and that rubber is what makes contact. Check the actual gaps at the strut and inside the fender, at full steering lock and full suspension compression, before ordering.