How this instrument works
Wheel offset is the distance between a wheel's hub-mounting surface and its true centerline, measured in millimeters. A wheel with more positive offset sits its face closer to the suspension side, tucking the tire further inward under the fender; a wheel with more negative offset (or less positive offset) pushes the face further outward, poking the tire closer to — or past — the fender's edge. Combined with wheel width, offset determines exactly where the outer face of a tire ends up relative to the car's bodywork.
Swapping wheels changes both numbers at once, and the two effects partly offset each other, which is why this calculator tracks two separate outputs instead of one. Clearance change tracks how much room opens up or closes off between the tire and the fender/suspension; position change tracks how far the wheel's outer face physically moves inward or outward. Both are built from the same two inputs — the width difference split in half (since width changes are centered, not one-sided) plus the offset difference — but combined with opposite signs, since a wider wheel with less offset can end up in a very different place than a wider wheel with more offset.
This matters directly for fitment: too much outward position change and a tire can rub the fender lip or stick out past the bodywork (both a cosmetic issue and, in some places, a vehicle-inspection failure); too much inward change and a tire can contact suspension components, brake calipers, or the inner fender liner. Checking clearance and position change before buying a new wheel-and-tire combination is the standard way to catch a rubbing problem before it happens on the road.
- Enter your current wheel's width in millimeters (or convert from inches: 1 in = 25.4 mm).
- Enter your current wheel's offset in millimeters — positive or negative, as printed on the wheel.
- Enter the new wheel's width and offset the same way.
- Read clearance change — positive means more room opens up; negative means less.
- Read position change — positive means the wheel's face moves inward; negative means it moves outward, toward the fender.
Worked example — going from a 7-inch to an 8-inch wheel
Swapping a current 7-inch (177.8mm) wide wheel at 42mm offset for a new 8-inch (203.2mm) wide wheel at 32mm offset: clearance change = (177.8-203.2)/2 + (42-32) = -12.7 + 10 = -2.7mm — a small net reduction in clearance. Position change = (177.8-203.2)/2 - (42-32) = -12.7 - 10 = -22.7mm, meaning the wheel's outer face moves 22.7mm further outward, toward the fender.
For a same-width swap with a much lower offset — 190mm width at 45mm dropping to the same 190mm width at 20mm offset, a common 'more aggressive stance' change — clearance change = 0 + (45-20) = 25mm (more clearance from the suspension side) while position change = 0 - 25 = -25mm (the wheel sits 25mm further out toward, or past, the fender), illustrating how a pure offset change without any width change still moves the wheel noticeably outward even as it opens up suspension-side room.
Questions
What's the difference between clearance change and position change?
Clearance change describes how much room opens up or closes off around the wheel and tire assembly overall — a mix of suspension-side and fender-side effects. Position change describes specifically how far the wheel's outer face physically moves inward or outward relative to where it used to sit. A wheel swap can increase clearance while still moving the wheel's face outward toward the fender, which is exactly what happens in a lower-offset, wider-wheel swap.
What does a negative position change mean?
A negative position change means the wheel's outer face moves outward, toward (or past) the fender edge, relative to its previous position. This is the direction that raises the risk of the tire poking past the bodywork or rubbing the fender lip during suspension travel and steering — it's the number to watch most closely when going wider or lower-offset for a more aggressive stance.
Why does width get divided by 2 in both formulas?
Because a wheel typically grows or shrinks in width symmetrically around its centerline — adding 25mm of total width usually means roughly 12.5mm more on each side, not all of it on one side. Halving the width difference reflects that a wider wheel's extra material is distributed both inward and outward from the original centerline, before the offset term shifts that centerline itself.
How do I know if a wheel swap will actually cause rubbing?
This calculator tells you the direction and magnitude of the change, but whether it actually causes rubbing depends on how much clearance your specific vehicle had to begin with — a stock car with generous stock clearance can tolerate a bigger position change than one already running close to the fender lip. Use this as a way to compare options and flag large changes for closer inspection, not as a guaranteed pass/fail check.
Is more positive or more negative offset better for performance?
Neither is inherently better — offset is a fitment parameter, not a performance one on its own, though it does affect scrub radius and steering feel slightly. The right offset for a given vehicle is whatever keeps the tire clear of the fender and suspension components while staying within the range the manufacturer or a reputable fitment guide recommends for that specific car.