For anyone else that's wondering, 50, 60, and 70 are NOT widths! They are PROFILES! In the case of the P-255/60R/15:
P=Passanger vehicle weight rating.
255=WIDTH, in mm, at the CENTER (about 10") About 8.5-9" at the tread.
60=PROFILE, the % ratio of SECTION height to with, about 6". To measure total height, add 2 sections (top and bottom) to rim diameter.
R=Radial
15=Rim diameter at BEAD.
So no matter what anyone says, a 205/60 is NARROWER than a 285/75! The 60 is simply a LOWER PROFILE tire!
And NO it did NOT change with the P-Metric system, an N-60 is still wider than an L-50! (For those who never heard of letter sizes, an N is about 12 inches wide, an L about 10 inches wide, and these sizes are still used on some drag racing tires).
Finally, there is no designation for tread width, all tires are measured across the center of the section.
Back to you Tom...