gas mileage problem...

ok, recently given an 88 firbird, the old 2.8 took a shit on the previous owner (my cousin) and he tore it down and apart and replaced the tyered out 2.8l with a 3.4l and re used the 2.8 injection system, doing this we had to have jet cut us a new chip, and it got roughly 225 miles to a tank (10 gallons used) of gasolien... seince his dui 2 years ago, car has sat, and it has a couple of expected problems that i have adressed and fixed already, but right now im getting 100 miles from full to empty... ive never drove a car with such horrable gas mileage, any one have any idea as to what i can be? no ses, no fuel leak, but the mixture smells rich, could it be a failing o2 sencor?? or something else? thanks in advance,
Doug