gas milage

shouldnt this 1991 5 speed xfi get about 45 or so mpgs mine is not what are a few things I could check to make the gas milage better

I have a geo 94 xfi and get 57 mpg on the highway, lost about 4 mpg because I got rid of the harmonic muffler and put on a turbo muffler, and put an slightly bigger air induction scoop, to increase air flow, giving it a bit more pep, but think what you should do, is check to make sure no vacuum hoses disconnected by the fuel injector(if you have a carburator it might be a bit different), don't put in platinum spark plugs, copper plugs seem to work better, new sparkplug wires, new rotor, a new pcv, new fuel filter, new oxygen sensor and make sure the tires are at the top end of the psi recommended, and then fill it up at same pump, letting the pump pressure kick the pump off, at a certain click, to give you an accuate highway testing of mpg, etc...leaning your mileage problem is probably related to the oxygen sensor, etc...

P.S. The only reason I increased the airflow to the car is that it wasn't idling well, seems idle on the 94 model is regulated by the pcv, car getting old needed a bit more air to idle properly, by getting rid of the harmonic muffler and installing a turbo muffler in place of the regular muffler downstream from the harmonic muffler, solved the idle problem etc...

Right

1st thing to do when your car runs like crap.... Tune up. Check the timing, replace plugs and wires, cap and rotor.