If you run a straight edge across the rim on the backside of your wheels (not the tires) and measure to the flat area where the studs go through; that measurement is your "backspacing".
On a 8" wheel the total width is measured between the beads inside the wheel, so it will usually be around 9" overall, but 8" between the beads. If that wheel was a 50-50 spacing it will have 4" outside from the brake or hub suface to the inner bead, and the same inside. However, backspacing is measured to the furthest inside edge that will clear any obstruction, so often it will be 1/2" further to the rim. So a 50-50 spacing on a 8" wheel will usually be 4" out and 4.5" backspacing.
Hope this helps.

 

ok so the less back spacing I have say instead of 4.5 I have 3.5 that will bring the wheel more inside the fender weld so it wont stick out and hit the edge of the fender?

 

yes, the higher the back spacing number, the more the wheel is tucked in the fender well.

 

went tire shopping today and nobody stocks front 235/60/15 and rear 245/60/15. I cant decide what to run, im pretty sure i like the 245's as they look a little taller than the 255 for the back, right now i have F-50's and a little wider than i like. Discount tires says the 235 should work on the front, but seems like most people are running 205 or 215?? not much selection either discount is BF Goodrich and expert tires are firehawks and they can only get 255 no 245's.

 

Haven't changed tires on the front of mine in years, but I'm running 235-60's on the front with 7" ralley wheels. They fit fine and have plenty of clearance. I got mine at America's Tire store.
BF Goodrich still sells 235/60-15 tires in the TA radials at Tire Rack stores. So does Goodyear and Hancook. I'm running Hancooks on my 1969 Suburban and they were very reasonably priced at $100 ea., with 85,000 mile warranty.

 

ok measured my wheels and they appear to be 8 inches wide and backspacing is like 3 3/4 so do you guys think the 235/60/15 will fit. I have been to several web sites some say thats good and some say 225/60/15. I want to be sure before i order.

 

Might occasionally rub if you hit a bump with the tires turned. The 8" wheel will have more sticking outside towards the wheelwell opening, so when turned about 45 degrees it will probably catch one of the inner fender bolts on the edge of the tread occasionally. A 7" up front would be better.

 

new wheels

 

 

Backspacing determines how far in or out the tire sits in the wheelwell. take off one of your front tires, lay it with the outside of the tire, the side with the valve down. get a straight edge and lay it across the rim. Not on the tire, but the rim, then get a measuring tape and measure from one of the bolt holes to the straight edge, thats your backspacing. It might be 4 1/2 inches or so. You can get rims with more or less backspacing to get the tire you want in the wheelwell. If you notice, those 4 slotted rims in the above picture, the rears have less backspacing than a normal tire-rim combination. Hope this helped you out. Bob