This is a very basic explanation so you can get the picture of backspacing.
Offset is how far the center of the hub is in relationship to the center/middle of the rim. You could have an 8" wide rim with a 4" backspace which gives you a 0" offset. You could also have an 8" rim with a 5" backspace by moving the hub center/middle deeper towards the outside edge (fender well side) of the rim -making the offset 1" from where the center/middle of the 8" rim which would be the 4". By going with a 5" offset, the rim/tire combo would sit deeper on the rear axle/front hub making the rim/tire closer to the frame. If you went the opposite way, the rim/tire will be pushed away from the frame and more towards the fender well.
Yep, some sites will have formulas, step-by-step measuring techniques, how to measure tire/sidewall width, etc. that to me only confuses everything. But this allows you to custom order your rim/tire combo in almost any configuration due to all the chassis/suspension modifications many of us do. I think the manufacturers also want to protect the seller/buyer if the buyer purchases the wrong rim/tire selection for his lowered, narrowed, disc brake conversion, custom chassis modifications and non-factory tires.
Factory rims were 4" to 4 1/2" depending on rim size, width, design, and type. So just lay a straight edge across the backside of your rim and measure from the straight edge down to the center hub where the brake drum/caliper would be resting. Key in selecting your rims is to make sure they will fit the front disc brakes. Generally not a problem with 15" rims and Pontiac certainly had 14" rims that were used on disc brakes, but earlier (pre-disc brake) rims will not work.
You also want to know what the tire width section is as compared to the actual tire tread width. You can have an actual tread width of 8" where the tire meets the road, but the tire bulges out at the side walls where they become wider. So, for example, an 8" wide tire tread might have a 9.5" or 10" tire width section depending on rim width. So tire section width may be a consideration when going to wider/fat tires.
Many rim/tire combos have been posted on here and a search will give you some examples. :thumbsup:
