Tire width on 8" rim

Does anyone have what the actual tire width measurement is with the following size tires on a 8" rim:

275/60R15 (advertised as 10.8")
255/60R15 (advertised as 10")

My dad has some Mickey Thomson Sportsman Pro 28x10.5 mounted on 8" wheels. They are advertised as a 11.2" sec width, but they actually measure 11 5/8". He let me borrow than just to see how they fit and I like the 28" tall tire, but the tire hits the leaf spring. So I measure the actual width and its wider than advertised so was curious about the 2 tires sizes above that I'm looking at getting.