The Dart came with 4½" wide wheels. The best you can get on these and get decent wear is 185/70-14. The Dart Sport 340 came with 5½" wide wheels. IIRC the backspace on these wheels is 4¼". The biggest tire recommended for a 14x5½ is a 215/70-14.
Well he did not say what particular Dart he had, so I measured my Dart 14" x 5.50" wheels minutes before I posted, and the back spacing is 3.75" as I stated above. Not an "IIRC" measurement. An actual measurement.
Yes 15x7, 4.25BS (=.25" positive offset), with 225/60-15 (26" tire) is a great combination on just about any Dart, and that is exactly what I'm running too.
Wheel diameter has no effect on tire width, wheel width, or backspacing. To maintain a 26" tire on a 17" wheel he would want to run 17x7, 4.25BS, and 225/50-17 tires.
