Charley, Your last sentence sums up the mileage issue. And maybe not, If the operating parameters of the engine are changed enough so that normal driving falls outside the most efficient rpms of the engine, the mileage will suffer. Example, radical cams don't work at 1500 rpm cruise. Too large a carb or intake, reduces air/fuel velocity and the mixture becomes less controlled. Both those items will usually help top end horsepower.
