If you want good fuel mileage then you should by a Prius. You can have a lot of power and ok gas mileage with a boosted v6. The stage II would get the same mileage as any other 90 degree Buick engine if it was the same combo. Using a stage block does not have an effect on fuel economy. If you want a solid low 11 second car plan on around 20 mpg highway and 14mpg city if your easy on it. Much worse if you have a heavy foot. Once you start changing cams and putting in looser converters the mileage drops off. Increasing the compression ratio can bring a small % back. Ive never built one with the intention of maximizing fuel economy. Driver technique, combo, and the a/f will have more to do with it than anything else. The combo and the persons tolerance will decide the level of driveablility.