Before I call Sonny at A&C the manual says
"offset" for tire rim selection. I think this might be a misprint. It doesn't say negative or positive offset.
On a 10" rim it says to have a 4" offset. That would make 9" of back spacing?
I found this Yokoham explanation.
Wheel offset
The wheel's offset is the distance from its hub mounting surface to the centerline of the wheel. To determine wheel offset:
Position the wheel on a flat surface and measure its overall width.
Divide the overall width by two, then subtract this result from the backspace value.
Offset = Backspace - (Rim Width ÷ 2)
The offset of a wheel can be one of the following three settings:
Zero offset:
The hub mounting surface is even with the centerline of the wheel.
Positive offset:
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front-wheel drive cars.
Negative offset:
The hub mounting surface is toward the back or brake side of the wheel's centerline. "Deep dish" wheels are typically negative offset.
Thanks
Ron