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...
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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