Popeye / Bob are 100% correct, the ET38s SHOULD fit no bother on a 4x4 Sapphire, but DO need spacers on a 2wd. This is NOT due to the offset, but the design of the wheel (ET38s would normally fit a 2wd). With the RS7s, the inside part of the spoke actually gets thicker as it approaches the outside of the wheel, which means this fouls anything other than sliding calipers (and therefore won't accept big brakes). The lower the offset (when referring to Fords) means that the FURTHER the wheel will stick out.

Therefore Pops is again right in saying that an ET35 wheel should fit in the same way as an ET38 with a 3mm spacer (ET38 - 3mm = ET35 offset).