Exhaust upgrades like yours are among many popular investments that, unfortunately, never pay back in fuel economy savings. You have probably seen plenty of other ideas for things to spend your money on that promise increased fuel economy, but they won't deliver either.
The quick, easy thing you can do is increase your tire pressure. Don't exceed the maximum marked on the tire, but do exceed the minimum recommended by GM. It's not huge but it's free and will also increase tire life and durability.
After that your options are driving style adjustments/techniques and more radical DIY modifications (mostly aerodynamic stuff, and potentially ugly).
Do avoid using the brake pedal (whose purpose is to discard energy; that's energy that you spend fuel to make and will spend more fuel to re-make) and try to keep RPM low. If there's a red light far ahead, brake a little bit immediately so you can cruise through a green light at a moderate speed instead of holding your current speed and completely stopping at a red light. Keep a long following distance so you don't have to do every wasteful thing the driver in front does. Plan ahead to ease up to curves and roll through them instead of staying on the go-pedal and then braking for the curve.
Of course reducing speed will help, but you didn't need to come here to learn that, so we'll assume you already drive as slow as you are willing. I don't think it's worth holding up traffic (though I do think obeying the law is, since tickets and the resulting insurance rates are so expensive).
One thing that is counterintuitive is that you shouldn't necessarily accelerate too gently. You want to get into higher gears in a shorter distance, and too-gentle acceleration with an automatic means it will go further in low gear.
The very best thing you can do, which supports ALL of this and anything else you might try, is to faithfully log your fuel economy. You can take advice all you want but until you have numbers and graphs showing trends over multiple tanks as you experiment, you can never know what works
for you and what doesn't. Start a gaslog here or at Fuelly.com and put every tank in it along with notes about what you did.
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