keep your car. Ive had my 5.0 since i was 17 (im 20 now). I usually get about 20 mpg city in the summer (17-18 in the winter) and usually 25+ highway in the summer. Ive done a best of 31 mpg highway, 70 mph cruise. Mines a 5 speed with 3.08s. The motor can do decently on gas if you treat it right. If it means that much to you, you can do what i did which is install a vacuum gauge on the dashboard or a-pillar somewhere. Its a real eye-opener. Try to stay above 10 inches of vacuum for commuting. I know that wont happen since youre 16 and floor it all the time to show off to your friends who all drive hondas, but for the 90% of the time theyre not in the car you should be able to handle driving responsibly. Also, reaplce the 02 sensors if you havent done that already. Mine really woke up after i did that and changed the fuel filter!!! Another free thing you could do (especially after fixing that vacuum leak) is to pull the battery cable so the car is dead for about 15 minutes, reconnect it and let it idle for 5 minutes. youll see the tach move around from 600-900 rpm ish as it optimizes from the default tune.
good luck and appreciate the things you have
PS im a junior in college now and own 3 cars. My insurance rates dropped a ton once i bought my 1983 F250 with a carb'd 300 cid I6! for the low low price of $300...My other car is my 1968 mercury cougar with the fred flinstone package. currently under construction while i neglect my poor 5.0 mustang since its the most reliable one i own.
TAKE CARE OF YOUR CAR AND YOUR CAR WILL TAKE CARE OF YOU