If you keep the stock exhaust, most of the power commandr maps actually take fuel away to increase the responsiveness. The software and several maps are available to down load for free. You may want to check them out before you buy, so I would think that would help, not hurt your gas mileage. The maps I have seen for after market canisters do add more fuel at certain rpms, the combination would probably cause your mileage to drop a little. I can email you my latest map if your interested.