MAN, my 95 full size, with high top conversion gets 13-14. You need to check the fuel filter, get a K&N air filter, and keep it clean and oiled. Here's the secret, you will gain 2 or 3 miles per gallon by getting a poweraid throttle body spacer, it's three bolts, just pull your throttle body spacer, clean up that and the manifold, and throw in the spacer with new bolts supplied. Your cat convertor is probably plugged, those years are famous for that. Just go to the muffler shop and have it replaced, mines gone. Have you ever swapped out the O2 sensor, it just screws in the exhaust right by the tranny. Maybe while you're messing with the air filter on the passenger side back of the throttle body carefully pull the clip off the air idle control valve and clean it, it just scews in the throttle body. Your 4.3 is basically the same as mine just short 2 cylinders. You can get all this stuff cheap at jcwhitney. That should get you started.