Why does my gas mileage suck???

I have a 00 se altima and my average mileage is about 18mpg. It runs fine but terrible gas mileage. I would appreciate all the help I can get!

Is the ECU storing and troubles codes?

go to autozone and test your oxygen sensor.
if its out, that's why.

also check your ECU and its operation.
if your oil ratio crashes in your ECU program
you get shitty gas mileage.

Any Time You Have A Vehicle That Is Getting Bad Gas Mileage And Has Been Tuned Up, If It Has Over 60,ooo Miles, The Oxygen Sensor Should Be Replaced. If You Read The Owners Manual, It Will Tell You They Should Be Replaced Every 60k To Keep Gas Mileage At A Maximum.
You May Not See The Service Engine Light On, & It May Not Run "bad" But If You Want Better Mileage It Needs To Be Replaced. I Know Its Difficult To Come Off Of The $40.00 To Replace It If Its Not Running "bad", But Trust,it Will Pay For Itself Many Times Over With The High Cost Of Gas These Days.