Ok, this is probably a very stupid question that could be answered in less than 30 minutes through a search, but too many jack and gingers in to perform that. if you have a tire that is say 12.5 inches on a rim of 7.5 inches wide, how does it create rubbing problems if you switch to a rim of 8 (or more) inches wide with the same backspacing, the tire won't get any wider at the ouside edges, is it the rims rubbing? thanks for your help with my drunken stupidity

 

A wider rim with the same backspacing as stock will stick out farther. The rubbing will happen because of this. As stock, the tires are more inside the fender, when the tires are moved out, the angle at which the tire moves with respect to the fenders changes. Make sense?

 

so it's the middle of the tire not the far outside measurement that matters, sweet, that's what I was looking for good stuff

 

The wider wheel will pull the tire outward slightly. If you are pushing fitment with a stock wheel, a slight difference in wheel width can cause rubbing.

 

Think of the centerline of the wheel and the centerline of the tire being aligned (have to be, it's the centerline). So, if you keep one edge still and move the centerline, the outside edge moves outward.

7" rim, 3.5" to each side of center. 12" tire , 6" to each side.
10" rim (same backspacing) 5" to each side.

If the offset is the same, we now move from the stationary edge (inside) - the 12 " tire is now centered 1.5" further out in the wheelwell. Rub point is at the front and rear of the wheelwell, hitting the tread, not the edge of the tire.

Tough to explain, wish I has pictures...

 

Thanks for the replies guys, that makes much more sense now