Rear tire size on a 75 Dart

Absolute largest you can go with stock spring locations is a 255, but backspacing will have to be perfect and the car itself will need to be at the upper end of factory tolerances and the same side to side (not all of them are).

245’s are a much safer bet given factory tolerances, and should fit most Dart’s if the backspacing is correct.

Assuming we’re talking about a Dart and not a Dart Sport. The sport body’s have more room.