I was looking at these rims but do not know what offset means. I have 4" backspacing right now. Is 4.125 in an 1/8 inch deeper than 4". What would the difference be on the front and back going from 4 to 4.125 in. Hope this post makes sense. Thanks.
Yes that would be 1/8 deeper...depending on the width of the wheel. Here's more info: http://www.maverickman.com/TireandWheelSizes.html You can also Search as its been covered hundreds of times.
It means it has 3.00mm of positive offset. To convert that to backspace you first divide the wheel in half. Then add 3mm(0.118 inches) to that, and you will get the total backspace. So if its an 8" wheel it would be 4" + 0.118 which would be 4.118.