Originally Posted by ///M Rakete
Tire manufacturer dimension markings are nominal values. You should always use the manufacturer's actual specifications for a given "size". According to Pirelli the 255/35-19 and 285/30-19 have the same overall outer diameter at 26".
You really shouldn't go by the "inches" that manufacturers round off to in their specs. It is an approximation. Remember that 0.1" =2.54mm. I have been measuring tires using the real millimeter dimensions for many years now, and you wont go wrong doing it this way, especially when you are looking at tires nearing the limits of proper fitment.
While different tire brands may be slightly different in their real dimensions for a given size, there is consistency within a particular model of tire. So unless you are using one brand of tire up front and a different one in the rear, the measurements should still give you the proper F/R tire sizes relative to each other.