F;

The tires are 28" outside diameter.
You'll need to check a tire chart to tell what you have now.

A 670X15 or 750X14 is 28" in diameter.
Bias ply generally work better for the early (pre 1970) cars & trucks, but many use 'radials' W/O problems.

If your happy with what you have, then there's no need to change.

Happy trails. \:\)

John M., I.I. #3370

"There are no shortcuts to any place worth going". -Anon