Picked up a set of M/T 315/50R17s today, looking for a cheap 17x9 wheel. My street wheels and tires are 18x9.5 with 275/55 18s. Truck is dropped 4" in the rear with shackles and hangers. I figure that I will measure the backspacing on my rear wheels and go from there. I have helper bags,so that complicates things concerning fitment.

 

Based on all the people who run Race Star truck rims with no fitment complaints and from Race Stars fitment guide:

Rear
15x10 = 7.00 BS (38mm offset)
17x9.5 = 6.23 BS (25mm offset)

 

An easy rule of thumb is 3.5" outside offset to tuck on a GMT800. Of course this will vary with tire choice and sidewall/tread stickout.

Although a 315/50/17 will fit on a 9" rim, I would be looking for a 17x10 with ~7" BS

 

I just got my weld s77 HD 15x10 rims in. They have a 7.5 backspace. I mounted them an put a strait edge up flat on the beadlock surface ( without the beadlock installed) and the straight edge fit about 1/4 inch inside the fender lip. I hoped on running 315’s but realistically i think i need to go 295 and i will probably trim most of that fender lip away. My 20” tires sit about 2” inside the fender so i absolutely need them to tuck and i know they will bulge out when the pressure is lowered

 

Interesting. Did you put a tape on the wheel to confirm the 7.5" BS? My 18x9.5 have 6" BS and with 295/45/18 have the same 1/4" inset from the sidewall bulge. The 15x10 Racestars have 6.625 BS and the sidewall bulge of 295/65 Hoosiers barely protrude.

 

Verified 7.5 backspace