Anyone have some good advice they can give me?

[firebirdvert305]

Well ive got a 93 laser turbo fwd....the car seems to run great....all thats on the car is a K&N air filter hacked aircan and turbo XS boost controller set to 12lbs of boost...Now onto my problem...i went to the gas station last week and fill my car upto F completly full to the rim. 2 Days go past and i see thats im 3/4's emtpy and i only went 188 miles Now 2 days ago filled up and (and i only use 93) and i took it too 3/4's full and today i look at the gas guage and its reading 3/4 emtpy and i only went 118 miles!! I mean ive been in boost but not so much to make my gas mileage this crappy. Im due for a tune up of plugs,wires, and fuel filter but come on can this really be the problem? The car idles great and runs great too.

All in all heres the breakdown:

Filled up completly went to 3/4 empty only went 188 miles...

Couple of days later....filled it upto 3/4 full then it went to 3/4 empty and i went 118 miles...

 

[firebirdvert305]

When i filled the car up with premium i remember it took like 15.6 gallons of gas which ran me like 31.00 bucks if this comes out to the right figure im getting 11-12 MPG

 

[AWDPetmitsu]

How old is the O2 sensor? When was it last changed? If it's been a while, I would start there first.