Backspacing: With the wheel laying on its face, put a ruler across the back of the rim (from one side to the other) and measure from the bottom of the ruler to where the hub mounts. This measurement is usually give in inches.
Offset: The offset is the difference of the hub mounting area in reference to the centerline of the wheel. A negative offset means the hub mounting surface is Xmm from the centerline towards the hub, where the positive means it Xmm from centerline towards the face of the wheel.
For instance: The Pro Comp 6001 HD 17X9 has a backspacing of 4.750" and an offset of -6mm
If you measure the wheels width it will actually be 10" wide due to the wheels lips each being close to an half inch wide, but still called a 9" in wheel. If you put the wheel face down on a blanket (to not scratch the chrome) and laid your ruler (ruler is just a way to get a reference point in the center of the wheel) accross the back, the distance of the hub from the ruler would be 4.750". Since the wheel is 10 inches wide, you know the centerline is 5". This means there is a -.25" (-6mm) offset. Negative because the hub mounting surface is closer to the truck than the centerline of the wheel. GMT400s come with a factory +offset wheel, so if you put a negative offset wheel on it, the wheel will stick out from under the fenders. One warning, wheels with a large negative offset will wear out the wheel bearings faster then normal, but I like the way they look.
