Just to clarify, because I think there's a lot of confusion in this area.
There is a difference between offset & backspace. Offset is the amount the mounting surface has been moved from the centerline of the wheel. Positive offset moves the mounting surface towards the outer side of the wheel, giving a shallower dish to the wheel. Negative offset moves the mounting surface the opposite direction, resulting in a deeper dish.
Backspacing is the measurement from the back edge of the wheel to the mounting surface.
As an example, a 7" rim with a 3.5" neg. offset would have about 1/2" backspacing. The 7" wheel is about 8" wide. 3.5" offset moves the mounting surface from the center of the wheel toward the brake drum 3.5". 8" divided by 2 gives 4" from the center of the wheel to each edge. 4" minus 3.5" offset = 1/2" backspace.
I think offset is a more accurate number because, when using backspacing, it is assumed that the total wheel will be 1" wider than the specified width. Whereas, offset always measures from the center of the wheel. If you know that the brake drum/disk wheel mounting surface is 1.5" from the center of the wheel tub, you know a 1.5" offset in the correct direction will center the tire. If you used backspacing, a 7" wheel would put the tire in a different spot than an 8" wheel with the same backspacing. If a 7" wheel with 1.5" pos. offset centers your tire the wheel would have a backspacing of about 6". For an 8" wheel to center the tire you would need a backspacing of about 7". This seems counter-intuitave because it looks like the larger wheel would move the tire closer to the spring because it has more backspace. You have to keep in mind that while the back edge of the wheel gets closer to the spring, the center of the tire stays in the same place.
I hope I did all of the math correctly. :read2: Someone straighten it out if I'm wrong. :munky2: