64 Dart, Want new wheels

The diameter of the wheels is not the issue. It's really about the width and diameter of the tires. As the wheels get bigger, you need to go to a lower aspect ratio tire to keep the diameter where it needs to be for the gearing and speedometer. I see people running 17" wheels with low profile tires, if you're into that sort of thing.

I don't really know what the limits are for these cars, although I'm very interested in what others have to say about that.

From what I've read, I know 205 mm width is safe. I think you can go up to a 215 mm width without problems. 225 mm is probably pushing it.

Overall diameter of less than 26" is a good place to be.

I'm planning to run 215/60/15 tires on 15x6 Cragars. That gives me a diameter pretty close to the factory size and about as much width as I can get without risk of rubbing.