Rear wheel/tire math problem, 69 Dart...
From my favorite wheel calculator. Ignore the 305 tire size, I did that to get the section width to match closer as it was suggesting a narrower section width for a 295. About a 1/2" lost to the outside according to the math, it underestimates the 255 too so it should be close...
Calculator is here if you want to mess with it, just pay attention to the section widths as they can vary.
Custom wheels, rim tire packages