Two names for one measurement
Offset and backspacing both describe where the mounting face sits relative to the wheel, and they use different datums. Offset is the distance from the wheel centreline to the mounting face, in millimetres, stamped as ET and positive when the face is outboard of the centreline. Backspacing is the distance from the back flange to the mounting face, in inches. Neither is better; they convert exactly:
Backspacing (in) = (stated width + 1) ÷ 2 + offset ÷ 25.4
The plus one is the flanges. A wheel stated as 7.5 inches wide is measured bead seat to bead seat, and the flanges add roughly half an inch on each side, so the overall width is about 8.5 inches and the half-width to the back flange is 4.25 inches. Add ET45, which is 1.772 inches, and the backspacing is 6.02 inches. An 8.5 inch wheel at ET40 comes out at 4.75 plus 1.575, or 6.32 inches.
The trap is right there. Backspacing went up by 0.30 inches while offset went down by 5 mm, and both are correct, because the width changed underneath them. Comparing two wheels by offset alone or by backspacing alone tells you nothing unless the widths match.
Work from the hub face, because it is the thing that does not move
The mounting face is bolted to the hub, so it stays exactly where it is regardless of what wheel is on it. Measure everything from there and the arithmetic falls out:
Inner edge, inboard of the hub face = offset + half the width
Outer edge, outboard of the hub face = half the width − offset
For the tire, half the width is half the section width. Running the numbers this page opens on: a 225 tire at ET45 puts its inner edge 45 + 112.5 = 157.5 mm inboard of the hub face, and its outer edge 112.5 − 45 = 67.5 mm outboard. A 245 tire at ET40 puts its inner edge at 40 + 122.5 = 162.5 mm and its outer edge at 122.5 − 40 = 82.5 mm.
So the inner edge moved 5 mm inboard and the outer edge moved 15 mm outboard. Two identities hold and the calculator prints both as a check on itself:
| Quantity | Figure | Why it has to be that |
|---|---|---|
| Inner movement | +5.0 mm inboard | Clearance spent toward the strut |
| Outer movement | +15.0 mm outboard | Poke toward the arch lip |
| Sum of the two | 20 mm | Exactly the section width change, 245 − 225 |
| Outer minus inner | 10 mm | Exactly the track change, twice the 5 mm offset change |
That is the part worth carrying away. Extra tire width has to go somewhere, and the offset decides how the extra is split between the two sides. Offset alone decides how far the whole assembly moves outward.
Track width goes the other way
Reducing offset moves the wheel centreline outboard, so track width increases by twice the offset reduction across the axle. Five millimetres of offset off each side is ten millimetres of track. It reads as a small number and it is the one that changes how the car behaves — scrub radius, the load path into the wheel bearing, and the leverage a pothole gets on the steering all move with it. That is a chassis engineering question rather than an arithmetic one, and it is well outside what this page will comment on.
What a static clearance measurement is worth
If you measure the narrowest gap on the inboard side today and enter it, the calculator subtracts the inboard movement and shows what is left. It is arithmetic on your number, restated, and it is the easiest part of the question. The tire moves through an arc as the suspension travels, sweeps across the strut as the steering turns, grows in diameter with speed, and sits closer to everything when the car is loaded. A gap that survives on a level driveway can disappear at full lock over a speed bump with four people aboard.
Work that does not forgive a mistake quietly
Brakes, steering, suspension, wheel fasteners, airbag and restraint modules, and anything holding the car up are in a different category from checking a tread depth. A wrong torque on a control arm bolt or a caliper bracket does not announce itself in the driveway; it announces itself later, at speed, and usually to somebody else as well. If you have not been shown that work in person by somebody competent, it belongs at a shop. Nothing here is a procedure and nothing here is permission.
Related pages
Diameter, speedometer error and revolutions per mile for a size change are handled by tire size comparison, and the tire size decoder reads a single size code apart. If the change in rolling diameter matters for gearing, gear ratio and speed covers that end. Once the new set is on, the tread wear calculator turns two depth readings into a rate.
Questions people ask
Is offset or backspacing the better way to compare wheels?
Offset, if you are comparing wheels of different widths, which is usually why anyone is comparing at all. Backspacing folds the width into the measurement, so two wheels with identical backspacing and different widths sit in completely different places relative to the hub. Offset separates the two facts, which is why it is what manufacturers stamp on the wheel. Backspacing is more useful when you have a wheel in your hands and a straightedge, because you can measure it directly without knowing anything about where the centreline is. The conversion is exact in both directions and this page will take either.
A lower offset gains me clearance inside, doesn't it?
A lower offset moves the whole assembly outboard, so on its own it does gain inboard space. The reason it often does not work out that way is that people change offset and width at the same time, and half the extra width goes inboard. Going from a 7.5 inch wheel at ET45 to an 8.5 at ET40 loses about 7.7 mm of space at the wheel flange even though the offset dropped, because half an inch of extra rim width on the inboard side is 12.7 mm and only 5 mm of offset came back the other way. Add a wider tire on top and the numbers move again. This is exactly what the calculator separates out.
How do I measure backspacing on a wheel I already have?
Lay the wheel face down, put a straightedge across the back flange, and measure from the straightedge down to the mounting face — the flat pad that contacts the hub. That distance is the backspacing. Measure to the pad itself rather than to the bottom of the hub bore or the back of a centre cap recess, because those are different surfaces and the error goes straight into every figure downstream. The stated width is bead seat to bead seat and is not the overall width across the flanges, which is why the conversion formula has a plus one in it.
What does the section width actually tell me about clearance?
Less than the number implies, because published section width applies at a specified measuring rim width and the same tire on a wider wheel measures wider than the label. It also describes the widest point of the sidewall rather than the tread, which is usually narrower, so the widest part of the assembly is not necessarily at the point you are worried about. Tread width, sidewall bulge under load and growth at speed are all separate figures that manufacturers publish per tire. Use the section width for the arithmetic and treat the answer as a first pass rather than as a clearance measurement.
Are spacers the same as changing offset?
Geometrically a spacer moves the mounting face outboard by its thickness, so a 10 mm spacer behaves like 10 mm less offset for every figure on this page. Mechanically it is not the same thing at all, because it changes what the studs are doing, whether the wheel is still located on the hub, how much thread is engaged in the nut, and where the load sits relative to the bearing. Those are the parts that hold the wheel on. This page does the geometry and takes no position on the hardware, which is a question for somebody qualified who can look at the specific vehicle, the specific stud and the specific spacer.