Given the subject of this thread, chew on this for awhile. Let's assume manual transmission. That means the engine rpms are directly related to vehicle speed--mechanical connection. Anything that increses horsepower means more fuel is processed for a given rpm/speed. Cold air intake--more fuel for a given rpm. Tuned exhaust--more fuel for a given rpm. If it gets more air, the computer will inject more fuel. Seems to me for FE the only effective modifications deal with how efficient the engine is. Lighter synthetic oil. Better engine blueprinting. More efficient accessories. Things like that.
Horsepower could make a difference if the drive ratios were changed to compensate. But, unless there are changes in tire diameter or differential ratio, it seems to me that more horsepower's not the way to go. This pains me 'cause I'm an old hot rodder.
I'm always late to the thread, but here goes anyway...
If the exhaust system needs to be replaced, go with stock if mileage is of concern not power. Nearly all vehicles have good exhaust systems when under light loads such as when driving for mileage, things change when you have high horsepower demands like when towing, as back pressure increases. Under high demand, energy is spent pushing the exhaust gasses out the pipe instead of propelling the car. Mileage improvements with exhaust systems are seen when the exhaust systems are heavily taxed with flow, this happens when you have high hp demands like in motorhomes, towing up hills, racing, etc. When driving for mileage in nearly all cases you will not have high hp demands - go with stock. If you are committed to a really light foot you can go smaller, just don't expect as much top end hp. Oh, and mandrel bent gives less turbulent flow reducing high flow system resistance.
For replacement systems I'm a fan of Walker DynoMax mufflers and stock pipe diameter. DynoMax has the flow directors for increased flow with reduced back pressure increase as flow increases, come in stock sizes and sound pretty good too, not loud and obnoxious. Alot of the auto-x ers use them, qualifies for stock in most if not all clubs, but flow much better than stock. I used stock pipe diameter to retain low end torque, but the most restrictive part, the muffler, replaced with one that doesn't constrict as much at high flow velocities.
For pure mileage, I think the ultimate system would be smaller than stock (size depends on engine hp demands), no muffler, smooth long radius bends and a 30" megaphone tuned to the most used RPMs, then more sharply tapered back down before the exit. For the street, replace the megaphone with a DynoMax.
Exhaust sytems have an increasing rate of resistance as hp output and flow increase. Bad systems have a rapidly increasing rate within the required flows, a large diameter pipe that reduces velocity, sharp bends that negate any tuning and increase turbulence. Good systems have a slowly increasing rate within the required flows, small diameter that keeps velocity high, smooth bends that keep turbulence to a minimum.
I read somewhere that he old benchmark Porsche 944 (naturally aspirated) actually performed better with SMALLER than stock size exhaust and a less restrictive muffler, the reason? Increased velocity at low RPM helping torque, and increased flow for a given pressure at the top end. Raw top end power was not as great as a larger system, but the system was designed for auto-x which values all-around performance, not the biggest advertisable splashy number.
Not to say all vehicles will respond this way, but my next system for the pickup will be smaller than stock.