It's not tread width, but section width.
It's tread width only if the particular tire has "square" shoulders like the real old Sonic Maxima tire used to have. (I.e., like a true racing slick)..
FWIW, they were just about the first really big, wide 60 series tire that was relatively cheap here in the mid-west back about '69 or '70 or so.
I'm talking L/60's and not too sure they weren't offered in either M/50's or N/50's too?? They were big WIDE mothers and looked great on the rear of my buddy's almost new Road Runner!!