A Flowmaster exhaust does improve gas mileage. This type of exhaust improves engine efficiency by taking away most of the restriction caused by a factory exhaust. Manufacturers produce standard mufflers to be more restrictive to make cars quieter. Restrictive exhausts also decrease horsepower.
Flowmaster is a company that started manufacturing exhaust mufflers to solve a growing problem related to sound issues at racetracks. Flowmaster produces exhaust mufflers for a wide range of vehicles from sub-compact cars to turbo diesel trucks. It manufactures chambered mufflers and laminar flow mufflers. Besides mufflers, Flowmaster produces catalytic converters, exhaust headers and complete exhaust systems.