rim backspacing

If the work were done you could come over and use some of my rims and tires to take measurements off of. You do have large bolt pattern, right?

Right now I am using 28x13.5 ET Street on Weld 15x10 with 7.5" backspacing. (Changing to 30" soon, so hurry!)
I also have 28x11.5 ET Street on Jeep 15x7 with 5" backspacing.