What is the offset on a steel wheel? The back spacing seems to be 4" but not sure of the offset.

 

With 5" rims and 4" back space, the offset (or inset) should be around 30mm. I think most Jaguars from the late 1950s through to X308 had that sort of offset as standard with steel and alloy wheels. Some wire wheels may have a reduced offset to give a wider track.

I think offset = back space - flange width - rim/2

 

FYI, you can go to most quality tire/wheel or performance shops and they should have the tool to measure your car's offset options. Even when you do not modify you car as much as I did, you often can get a wider wheel/tire if you measure and get a custom offset wheel which are also available in stock popular styles like the Dunlop racing styles that used in Jag racing days.....