305's on an 11" rim

Did you ever mount the 305/45r17's? I am having a hard time deciding on 315/35r17 or 305/45r17"s. I like the 28" diameter tire and a little more sidewall but you lose almost 2" treadwidth (nittos) to the 315's
How do you figure 2" loss? Theoretically, the 305 should be less than a half inch narrower in section width. Is the tread that much narrower on a nitto?
Personally, if i have room for the height, I'll take the taller tire every time!