Offset locates the mounting face
Offset, often marked ET, is the distance from the wheel centerline to the mounting face. Positive offset puts that face toward the outside of the wheel. With the mounting face fixed to the vehicle, a lower offset moves the wheel center outward.
The width model
Convert the stated wheel width to millimeters. The nominal inward distance from the mounting plane is width ÷ 2 + ET; the nominal outward distance is width ÷ 2 − ET. These distances refer to the stated rim width, not to the outermost edge of a rim lip or an inflated tire.
8 in ET45 to 8.5 in ET35
The original width is 203.2 mm and the new width is 215.9 mm. The inward distance changes from 146.60 to 142.95 mm: 3.65 mm less inward reach, or 3.65 mm more nominal inner space. The outward distance changes from 56.60 to 72.95 mm: 16.35 mm farther out.
Position is only part of the fitment question
The calculation excludes tire bulge, rim lips, spoke and brake shape, bolt pattern, hub bore, fastener requirements, suspension movement and body clearance. A positive inner-space change is a geometric result, not evidence that a wheel clears the vehicle.
Sources and further reading
- What Is Wheel Offset? — Method Race Wheels