Offset measures where the mounting face of the rim is located in respect to the centreline of the rim. +38 means that it is 38mm from the centre line.
Eg a +48mm would have the wheel sit 10mm further inside the guard (compared to +38 for same wheel width).
+28 would make it sit 10mm further out from the centre of the vehicle (and might risk it rubbing the outside guard).
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