Offset is a little different than backspacing. Here's a formula to determine backspacing if you have rim width and offset:
1. WHEEL WIDTH + 1" & DIVIDE IN HALF = X
2. DIVIDE THE OFFSET BY 25. 4 = Y
3. X + Y = BACKSIDE SETTING
8" wide with 0 offset = 4 1/2" backspace. Wonder what the rims are rated at for load?
Brad