Hmm,.. lets see if i can math this out,...
If the rim is 7in wide, and the back spacing is 5.25in, and offset of the measurement from the center of the rim(width wise) to the mounting surface of the rim,...
7 / 2 = 3.5in to the edge to the center of the rim, width wise
5.25(backspacing) - 3.5 = 1.75in
So the mounting surface of the rim is 1.75in(1 3/4in) off to the outside of the rim. Offset is measured in MM, so 1 3/4in converted to MM is 44.45mm
sound about right?