What Dan said. Rim width measurement is between the bead seats, not overall. Confirm that's where you're measuring the width. 15x5.5 rims were optional on a '53-55.2 3100's. Standard rims on '56 3100's were 15x5. Later years used the same rim dimensions, but the hubcaps fit over the bumps on the inner ring like Dan mentioned. The 6x5.5 bolt pattern was used pretty much throughout the 1/2 tons into the 70's.
