There tends to be plenty of confusion between "offset" and "backspacing". Both terms are related to each other, and along with wheel width, are dependent on each other as well.
Backspace is the measurement from the wheel mounting surface to the inside lip of the wheel. On these old trucks, 3 to 3 1/2 inches is the common size, it can vary depending on the width.
Offset is the dimension from the wheel mounting surface to the wheel centerline, usually measured in millimeters. If it is on center, then you have 0 offset. If the wheel is off center to the outboard, that is positive offset (like todays FWD vehicles), and offset inboard is negative offset. To get the proper offset, one has to know the desired backspace and wheel width.
To give an example, a 15x8 wheel with a 3.5" backspace for a typical Chevrolet truck ('73-'87 1/2 ton 4x4) will have a -25mm offset.
