Ordered a gear vendor, help with deciding gears

Top gear o/d ratio would be great at highway speeds. Any time you can get the cruise rpm at or below 2000 at cruise speed you will get good mileage and lower rpm is easier on engines and makes them last longer. Just the way I think about it. Now if you build a 450-500 horse small block you won't be getting good fuel mileage regardless of rpm. The whole combination needs to work together.