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Smaller exhaust?
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.
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(just read his log and he wrote that it is getting 25 to 27 mpg)
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