Start at the mounting plane
The drawing fixes the hub mounting plane at zero and calls the outboard direction positive. A wheel with positive ET places its centerline inboard of that plane. Subtracting ET and adding any spacer gives the centerline position used in the calculation. Both baseline and candidate are evaluated against the same reference.
Wheel bead-seat edges are centerline plus or minus half the nominal bead-seat width. The program reports inner intrusion as the baseline inner position minus the candidate inner position. A positive result means the candidate reaches farther inboard. Outer protrusion is candidate outer position minus baseline outer position, so positive means farther outboard.
Width and offset act together
Increasing wheel width adds material on both sides of its centerline. Reducing positive ET shifts the whole geometry outward. Those changes can partially cancel at the inner edge while adding at the outer edge. Looking only at the offset difference misses the width contribution.
For example, adding half an inch of bead-seat width adds 12.7 mm in total, or 6.35 mm per side. Moving the center outward by 5 mm leaves 1.35 mm additional inner intrusion and 11.35 mm additional outer protrusion. These are nominal bead-seat movements, not measurements of a specific wheel lip or spoke.
Keep the reference surfaces distinct
A wheel’s overall width includes features outside its bead seats. The program does not invent a universal lip allowance, and it does not calculate actual backspacing from an assumed flange profile. Tire section width is also not the same as wheel width.
Use the tire changes for the optional tire-envelope gap arithmetic and the wheel changes as a separate dimensional comparison. A brake caliper can conflict with spoke geometry even when both nominal edge calculations look acceptable. A real compatibility assessment needs the relevant manufacturer dimensions and the actual vehicle.