I am wanting to widen my factory gmc 20's 1.5" to make them a 20X10. Of course the extra width will be added to the inside of the wheels. First, will that fit? I have measured and it seems that is no problem. Biggest part is can i fit 315/35R20 toyo proxes on the rear without rubbing. Again this is all only on the rear tires

 

Pretty sure the stockers are 7.5 so you need at add 2.5 to the inside, and no, they will not fit without a spacer.

 

The stock 20's measure out at 8.5" wide. Spacer as in moving the wheel further out? That might look bogus....

Plus guys here seem to fit 305/50R20 tires on factory width wheels without rubbing and that is even more confusing because thats a much taller tire...?

 

Ah, the 16s and 17s are 7.5, I guess they changed it for the 20s. Yes, move it further out. When I had my 17s widened to 10" the rim would hit the leaf spring, not even including the tire. Width is much harder to fit than height.

 

Still looking for a definitive answer on this widening of wheels and 315 tire fitment before i spend a bunch of money... Appreciate any opinions/experience

 

seems to me you could measure from your hub to your leafs and see how much room you have to grow?

 

From the leaf spring to the factory tire edge is approx. 3in To the wheel edge is approx. 5in

 

sounds like you have room to grow

 

the 315/35/20 fit without widening them
https://www.performancetrucks.net/fo...2012-a-494470/

and stock 20s are 8.5"
http://www.finishlinewheels.com/spec...28SET+of+4%29/