I used to run my 400 with the RA IV cam, auto and 2.56 gears and got 18 mpg at 70 mph. The same engine was installed into a four-speed '77 TA with 3.23 gears and got about 15 mpg.

If you want to keep the same acceleration off the line after changing to a much bigger cam, usually the difference needed is going to be at the most a 20-25% increase in ratio if the engine is tuned properly. That means that a 2.56 should be changed to a 3.08 - 3.23 for example.