Don,
Note that "85" series tires are slightly narrower in cross-section than "75" series tires.
What that means is me going to series 85 tires still gives me plenty of spacing between the rear duals while at the same getting me a larger tire diameter for a taller overall vehicle gearing ratio so as to help counter the E450's lower differential mechanical ratio.
I wanted to not disturb spacing between the duals for two reasons: So they wouldn't rub, and so air circulation between them in hot weather wouldn't be negatively impacted.
