where is my gas going?!?!?!

hello all,i have a 1993 nissan sentra se-r, with auto tranny. well it had a hole in the exhaust flex pipe, but i fixed that. but my gas is draining like no other.im getting worse gas mileage than a 305 camaro. its a 13 gal tank and only getting 160 miles per tank. and i used to be able to take a 60 or so around trip and the needle would barely move. and now it drops fast. and my tranny is slipping bad, but it was slipping pretty bad when i was getting good gas mileage. and i noticed while i was working under the car on day, i heard the fuel pump kick on for a couple of secondswhen the car wasnt running, it didnt even have the key in it. any suggestions? by the way, will the front sway bar pop if you turn on it without a bushing in it? yeah, i think im missing a bushing. and the last problem im having is that when i go and start it up in the morning it vibrates very bad, until it warmsup. but then it still doe's it when your going down the road, but only when you let of the gas pedal.but if you accelarateit will stop vibrating. any help would be greatly welcomed.

maybe i should stick to working on the old cars.

any idea's?