If your mileage is that low something is definitely wrong.

Full Tune up - plugs, wires, all filters, fluids, etc

Tire pressure

Any CEL's? You'd think a bad O2 would trip something being it's OBD-II.

Chip, intake, & exhaust mods all help MPG.

2000 SVT #674 13.47 @ 102 - All Motor! It was not broke; Yet I fixed it anyway.