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Increasing your cam duration / lift will move your torque curve up in the RPM band thus raising your HP. It will certainly reduce your mileage. High-duration cams have a longer period with the intake and exhaust valves both open. It drops cylinder pressure and wastes gas as the fuel/air mixture flows from your intake to exhaust when both valves are open. This flushes out the exhaust gases from your cylinder, resulting in more power. You will also need to ensure that the rest of your components match your cam if you change it dramatically (head ports, valve size, intake runner length, header length, injector size, computer program, gearing, torque converter slip, etc.) I would be careful about making a large change in cam design in a modern engine.