15.0 mpg to 21.6 mpg!

360 / 727 / TTi headers, X-Pipe, Borla Pro-XS (str8 thru) / 3.55 open diff. Will be procuring an Eaton True-Trac after tires. But I can't decide whether to stay with the 3.55 or go smaller (in the direction of a 3.23 or beyond).
With that combo, 13's is a sleep walk (providing you hook up). 12's if it has around a .470 lift cam and something for an aftermarket converter.