Comparison of my wire and Rostyle wheels. The outer face is the closest comparison between the two, giving a (near) standard clearance to the outer arch, which has the effect of reducing the track with the wider Rostyles. If choosing wider wheels you would have to go for more offset to maintain that clearance, while still giving adequate clearance to the inner part of the wheel well.
Positive, zero and negative offsets, where the mounting face is outside the centre-line of the wheel. Positive offset is the usual arrangement - for steel and alloy wheels at least, to bring the angle of the kin-pin and the centre-line of the tyre together at the road surface. Negative offset is often used on show cars to bring the wheels out, but it causes them to move back and fore as the wheel is turned instead of pivotting and can adversely affect handling. Image from Crankshaft Coalition.
Offset and other wheel dimensions, from The MG Experience
