Why do i only get 200 miles to a tank with a 4 cylinder?

i have a 93 plymouth sundance with a 2.5 liter 4cylinder engine with a 4 speed auto . i've done the calculations and it showed that i'm only getting 14miles to the gallon. i have a 14 gallon tank, and i dont drive it rough at all but most of my travelings in the city. i have done a tune up and everything to try and find out why my gas mileage is so low. I've changed the o2 sensor the MAP sensor the plugs, wires, fuel and air filter and pcv valve within the past couple miles and still getting bad gas mileage. i've tried doing trouble codes with the engine light myself and nothing shows up. i would take it to a shop for the codes but they charge $40 for it because of the older computer. my tires are at 40psi so i don't think its a tire inflation problem i've been using fuel system cleaner but i havn't used any the past couple fill ups. i have about 80000 miles on my car. please help me i need to know whats causing this kind of mileage.