Reading inner and outer wheel position

Keep the mounting plane fixed

Imagine the vehicle’s mounting face as a vertical line that does not move between the two drawings. A wheel extends inward and outward from that line. TireSpan measures each nominal width boundary relative to this common reference, so the comparison does not drift when the wheel width changes.

Read the inner result with its definition

The inward distance is half the converted width plus ET. If that distance decreases, the inner width boundary reaches less far inward. TireSpan describes the opposite of that difference as more nominal inner space. The wording refers to a geometric reference; it does not measure the gap to any particular vehicle part.

Read the outer result independently

The outward distance is half the converted width minus ET. If it increases, the outer width boundary sits farther out. A wider wheel can move both boundaries even when ET does not change. An offset change can combine with that width change differently on each side.

Work through both sides

For 8 in ET45, the half-width is 101.6 mm: inward distance 146.6 mm and outward distance 56.6 mm. For 8.5 in ET35, half-width is 107.95 mm: inward distance 142.95 mm and outward distance 72.95 mm. The result is 3.65 mm more nominal inner space and 16.35 mm farther out.

Know which boundary is drawn

A stated wheel width is not the overall width including rim lips. A mounted tire can extend beyond the rim-width reference. Tire bulge, wheel flange geometry and spoke shape are outside this model. Do not use the diagram as a scale drawing of the complete assembly.

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