[Mike Bradbury]

Offset and backspacing are another confusing kettle of fish because they are also a mash up of metric and imperial values.
offset is measured off the centerline of the wheel. Negative gives you more "dish" versus positive offset pushes the hub of the wheel more toward the outside of the vehicle. Offset is measured in millimeters

Backspacing is the distance from the hub to the inside edge of the wheel and is measured in inches. This value is important especially if you are running small 16" wheels because components like disc brakes and lines may get in the way with too much back spacing.

Measure your current wheels while they are on your truck. Turn your wheel to the left or right, and with a tape measure measure the distance from the inside edge to the closest component it would hit if it had a deeper backspace. Remember the frame could also get hit by the tire with a ten inch wheel. Then go on the other side of the vehicle keeping the wheels cranked to one side, and measure the other side for interference issues. Next jack the truck up and take off the wheel. put a straight edge across the inside of the rim and tape measure from the straight edge to the inside hub of the wheel. This will give you your current backspace. Next, with your measurements you took while it was still on the truck you can estimate your new value. Remember tires flex in turns so don't make the backspace and off set to tight of a tolerance because it may rub when you are moving.