If you're looking to increase your gas mileage, Magnaflow might not be the best route. While aftermarket exhaust systems can improve engine efficiency, they typically don't offer a significant boost in fuel economy. In fact, most performance-oriented exhausts are actually geared towards increasing horsepower and torque rather than improving gas mileage. That being said, if you install a Magnaflow exhaust system on a highly efficient vehicle, you could see a slight uptick in fuel economy. But on the whole, don't expect your gas mileage to improve significantly just by installing a new exhaust system.