What's the biggest tire you can fit on a 73 swinger

Figure out what the most backpacking you can get it and then go from there.If you are running a 15 in wheels anything over a 275 is hard to find in a street tire mostly soft compound tire. you can always lift the back of the car a 1in and let about a 1in of tire stick out if needed.

You could run a 275 tire on a 15x8 rim with about 4.5bs and still be inside the wheel well.

Or

You could run a 305 315 tire on a 15x10in rim and have a little sticking out.