How much fuel economy does flowmaster exhaust system add?

A Flowmaster exhaust system will generally reduce fuel economy, not improve it. While some minimal gains might be seen in *very* specific circumstances (like a heavily restricted factory exhaust), the overall effect is usually a slight decrease in MPG. The amount of reduction varies depending on the specific vehicle, engine, and Flowmaster system installed. You're unlikely to see a significant change (more than a couple of MPG), but a small decrease is more common.