I didn't think it measure exactly but I did test fit the 17x9 rear (5.250" backspacing) with 255/50/17 Nittos on the front.

I did so without the 1/4" Wilwood spacer and I have Hotchkis upper and lower a-arms, no spacer with the Hotchkis big block springs, and my GW solid sub-frame mounts that lower the body down 1.5". So there are a lot of variables on my setup.

With my camber set to -1 degree, the tire cleared into the fender just enough to not touch the outter bolt heads that attach the inner and outer fender. I turned the steering lock to lock and found no interference aside from where the tire meets the actual sub-frame, which happens with the 17x8s. They seemed to fit just as well as the 17x8 with different backspacing, tire size, and with the Wilwood spacer.

When my new tires wear out, I plan to buy 2 of the 17x9s to put on the front.