[comptiger5000]

A re-done exhaust properly tuned to optimize flow and power at lower RPMs can help mpg. Just any higher flowing system, however, may or may not.

For example, appropriately sized long tube headers of the correct length, followed by a high flow cat and decent flowing muffler, with appropriately sized (read: not as big as you can fit) piping can definitely help. The gains may not be significant though.

John - if the exhaust system is made too large, it's poorly tuned for low rpm, and will have turbulence and poor flow that doesn't clean up until much higher rpms.