[Geo Metropolis]

It seems the general consensus has been to choose belly pan or air dam, not both, so on my metro I only did the belly pan and saw good improvement. Then after a while I added an air dam and saw even more improvement in mileage and especially handling. The metro has a high stagnation point and forces a lot of air to pressurize under the car which led to unpredictable handling with the belly pan alone. By adding an air dam that lowered my stagnation point to the same level as the belly pan, air is no longer pressurized under the car. Instead, the majority of air is pushed over and to the sides the rest slips easily under the car with the belly pan - my handling has improved dramatically and I've seen good mileage results.
I suspect you would see a good result with the belly pan, and an even better result if you add an air dam. I originally tried to make my air dam really low which was a mistake as I busted it to smithereens. It seems that belly pan height is perfect and this allows it to clear curbs, driveways, and off road situations easily. I suggest you angle it like a wedge and curve it toward the sides of the vehicle as smoothly as possible so it forces air up and around the side of the vehicle. Cut appropriate grille openings if you cover the grill (I had to adjust mine a few times making bigger grill openings to accommodate high desert temps).
I've chronicled my build really well but gotten behind a bit so I don't have a writeup on my front air dam yet, I'll try to post it up soon.