Fuel issues

I'm either getting very bad gas milage (worse than what my computer says, which is still low at 16.8 mpg), or fuel is somehow leaking without me seeing it, or someone keeps coming to my car and siphoning fuel. Right now I'm getting a mere 175 miles out of a whole tank. I am also having an issue with my cylinder 5 misfiring, is it possible that this issue is hand-in-hand with my terrible milage?

There are many possibilities. The misfire will not usually cause mileage to go down but anything is possible. Personally, I would change the O2 sensor, upstream, and see if the mileage improves. If you have a lot of unburned fuel it can contaminate the sensor and cause the car to run rich. Even without a CEL, I would address the misfire and then change the O2 sensor.

I've checked the plugs and injectors and they look good, I'm taking it to the mechanic tomorrow to do a bunch of work (belts, intake gasket, tune up), I'll see what he says. It would suck if it was all fixed up and still got 175 miles per tank. I don't feel like it's running rich..it doesn't smell but I guess that doesn't rule out the possibility.

My car is also dinging at me now (like the ding to alert you that you're running out of gas when the low fuel light comes on), but there's no light on my dashboard display. I checked all of the lights and they all work.

I would hold off on all the work (especially expensive LIM gaskets) untill the missfire is figured out. Do you know the previous history of this motor? Do you have a stock pully?

Have you done a compression check on it?

Tell me, how did you "check the injectors"? If you know the missfire is on number 5, I would swap the #5 injector to a different cylinder and see if the missfire follows. I have seen fuel injectors stick open causing a single cylinder to flood with fuel.

Have you did like Bob (bnaylor) suggested and ohm checked the injectors? What were your results?

I was thinking about holding off on the work but..i don't know something's telling me that this work will fix it for some reason. Yes I have a stock pulley, and the guy had records of all previous work done on the car...unless he hid something, it seemed like a car that had been taken very good care of. By checking the injectors/plugs all I did was a visual inspection/gap test, I don't have the resources anymore to do compression or ohm checks. I didn't bother switching the injector (although I really should have), because my check engine light goes away real quick, it was just a fluke (if you want to call it lucky, hah) that the light stayed on long enough to check for codes that one time.

Edit: After the tune-up the problem has so far gone away. My fuel going down while not driving problem still seems to be partially there..I'm guessing it's just a weird fuel gauge because there are no leaks under my car.