changed sparks plugs,got a little better but still having bad gas mileage. on 1994 Chevrolet Corsica

I didn't change the wires.I cleaned the filter, and changed the spark plugs.the mileage got a little better,but it still seems to be bad. does distributor cap have anything to do with that? I din't check to see if it is burnt.I do't know how to find these diagnostic codes people write does a person retrieve that?maybe that could help us.

Asked by for the 1994 Chevrolet Corsica
It's been awhile, but if you can get at plug wires,changethem. Not sure if there's a cap an rotor to replace..poor mileage... has the fuel filter been replaced? As far as the codes are concerned, if the check engine light isn't on, there's no code to retrieve