93 should have a one wire O2 sensor. You can swap it to a 4 wire from a junkyard pretty easily with just a minor bit of wiring. That will give it a faster startup and keep it working even at idle when the one wire cools off and stops working. It is a slight improvement over the stock one wire and is worth considering when having to replace it.
4 Wire Oxygen Sensor conversion - MetroXFi.com
Just replacing the one wire sensor with a new one is normally pretty cheap. Last one I bought was like $15.
I would guess 45mpg is normal for an easy driven base 5spd. So first thing is do a compression test. It should be over 175 or so. If one cylinder is lower than the others then it probably needs exhaust valves. Just get the stainless ones from 3tech and they will never go bad again.
If compression is ok then next is O2 sensor. You can take it off and test it with a propane torch and voltmeter as listed in that link instead of just replacing it.
Next I would check the brakes/wheel bearings. The rear tires should spin forever about like a bicycle when you jack the back end up and spin them. If the brakes drag or the bearings are bad it won't spin real easy. The front should spin pretty easy but the disk brakes have a lot more drag than drums so it won't be quite as good. An easy way to check them is to jack both wheels up and let it idle in 1st gear and see if it favors spinning one wheel over the other. Get full loaded calipers if one of them is dragging. It is usually the slider pins that get stuck and are very hard to get working properly again without just replacing the whole thing.
Those are the main things I can think of that cause bad mileage. Hopefully you can get it back up to just under 50mpg with easy driving and keeping it 55mph and under.
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. Once you feel comfortable with your driving technique, you can move on to areo-modding (if you do long highway commutes) or tuning of the engine (in general, slightly expensive). Good luck and welcome to Ecomodder.
