Will,
Not knowing what kind of gearing you have in the rear-end or tranny, it would be kind of hard to guesstimate what kind of mileage you should expect.
I can tell you my trucks, one 235, one 216, and one 250 I-6's, all with original 4 spds, and 4:11 rear ends will barely see 10 mpg on a good day.
