Howdy,

What are the reference points?

Take a yard stick along with your measuring tape.

Offset is the distance in inches (or millimeters) that the mounting surface is offset from the wheel center line.

Offset is related to backspacing. Backspacing is the distance from the back edge of the rim to the mounting surface.

Measure the width of the wheel rim from bead edge to bead edge; measuring the tire mounting width. Divide by 2. Keep that number in mind; that is the wheel centerline.

Lay the yardstick across the back edge of the wheel. Measure from the straightedge to the mounting surface. That is the backspacing. Subtract the smaller number from the larger number. That is your offset. If the backspacing number is smaller than the center line number, the offset is negative. If the backspacing number is larger than the center line number, it is positive.

If the wheel you want has a tire mounted, you will have to estimate backspacing as closely as you can. Lay the wheel and tire face down on ground, lay the yardstick across the tire. Measure from the yardstick to the ground. This number will be the tire width across the widest part.

Measure from the yardstick to the bead. Double that number and subtract from the tire width. Divide the remainder by 2. That is the (estimated) wheel center line.

Now measure from yardstick to the mounting surface. Subtract the distance measured from the yardstick to the bead. That is the (estimated) backspacing.

Offset is determined by subtraction just as before except using your estimated numbers.

I used "estimated" because there are all manner of errors introduced in this indirect measurement. It is close enough to make a decision about what wheel will work and which one won't; but it will probably not be exact.
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)