I just picked up a couple weeks ago a 1986 F250 Lariat XLT with a 460ci and a 3speed C6, and so far i have got about 15 to 16 consistently. Those are almost all highway miles but for a carbed 460 its super great, i dont know the rearend gear ratio yet but i thinks it must be 3.55 or close to for this good of mileage