I recall Tim Engel saying that Spitfire wheels (for instance) have a different offset.
Width. x Offset
4.500" x +1.062" (+26.97mm) Stock Steel Wheel
5.500" x +0.637" (+16.17mm) Optional 'Brand Lotus' Alloy
4.500" x +0.8125 (+20.64mm) Triumph Spitfire Steel Wheel
h/t Tim Engel