Quick question - I think I know the answer, but I wanna make sure. The higher the offset means it sticks out further correct? So +15mm offset will set in the wheel well more than a +18mm offset.
A confirmation would be nice.
A positive offset means the the wheel, when bolted to the car, is more INSIDE the fender. A negative offset means the wheel sticks more OUTSIDE the fender. (And depending on spoke design, a negative offset wheel tends to have more of a lip on the rim.)
Offset is how far the hub mounting surface is from the middle of the wheel.
A Zero offset would mean, for example that the hub mounting surface is exactly in the middle of the width of the rim.
A +25.5 offset (25.5 mm = 1 inch, BTW) means the hub mounting surface is 1 inch towards the OUTSIDE of the rim. Try to visualize putting a wheel like this on a car. It means more of the wheel is under the fender. The opposite holds true for negative offsets.
Hopefully this helps more than confuses. It took me a while to get the hang of it.