Answer for Matt Cartney:
If everything is stock and you put on a freer flowing exhaust, fuel economy often goes down. With a restrictive exhaust, less of the exhaust actually leaves the motor each cycle. This is just like "exhaust gas recirculation." The second time the cylinder fires, there is still some unburnt fuel from the first time it fired, etc.
To get the same mixture as stock, an opened up exhaust needs to be made richer, to make up for some of the fuel not getting a second chance to burn. Even without being made richer, more unburned fuel is leaving out the exhaust than in the stock condition. While you have the potential for making more power, and perhaps that would make the difference in high wind or up steep hills, for regular riding the wasted fuel outweighs the better breathing of the motor and fuel economy goes down a little.
This is one of the reasons that economy cars get little tiny exhaust pipes. The car would perform better opened up, but they want the restriction because it helps fuel economy.
Matthew
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