2000 Plymouth/dodge neon getting TERRIBLE gas milage,(9.3 miles to the gallon) engine shakes?

the car starts and runs fine, put it in reverse, or drive, it shakes when you put it in gear(idling).. but if your driving, stop at a stop light, it doesnt do it.as soon as you do the park to drive thing, it shakes again at idle) has new motor mounts. and another thing, i just got this car, i put $20 in it from E (9 gal) put it over 3/4 of a tank, (after it settled) i drove 28 miles, and its setting on half tank, it was city miles, but this car "should" get 28 city miles a gallon,and from my figuring, thats about 3 gallons it used in 28 miles, making it get 9.3 miles to the gallon, at least thats what my research says. the car runs fine, i am considering new plugs, air filter is clean, oil is clean(probably 1000 miles on it). i put fuel injector cleaner in the gas, says it will help with gas milage(other than cleaning the injectors), but i did this the first time i put gas in it. you can smell the exhaust when you first start it, and its running rich, any ideas? thank you all, and if you need more info, i will provide it. thanks!