Backspacing still doesn't make sense to me.
I don't know about your car specifically, but here's some info on how to measure offset.
Offset
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 front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly.
I don't know about your car specifically, but here's some info on how to measure offset.
Offset
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 front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly.
