From Baxman, Grimstad and others:

I can share stock specs:
Stock tires: 245/70-16 or 245/60-18, both 29.5" tall and about 9.5" wide.

Backspacing (distance from mounting surface of the axle to inboard edge of rim) is 5.25" or 140mm.

Backspacing is directly related to offset.
Offset is the distance from the mounting surface of the axle to the center of the rim, which is +38mm stock (or positive 1.5").

And

Offset Explained:
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

Zero Offset:
The hub mounting surface is even with the centerline of the wheel.

Positive:
The hub mounting surface is toward the outside of the wheel.

Negative:
The hub mounting surface is toward the inside or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

WARNING:
If the offset of the wheel is not correct for the car, the handling can be adversely affected. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. (Edited by Roy)