I'll throw my 2 cents worth in here because I kind of have these combos.
My 70 Challenger with 3.23 and a 225/70R14 rear tire runs 3300 @ 70-75 mph.
My 68 Charger with 3.23 gear and a 4 speed od out of a pu truck with 275/60R15 runs low to mid 2k @75 mph. Gets mid teen, mpg from a 440, all...