I just filled it up and I'm getting 13mpg
Mind you this is about 95% city driving but its still seems really bad

What is the average mileage city/hwy?

Is there any thing I can do to improve it?
I checked the codes and there was nothing stored

Any help on this would be great
Thanks in advance,

Air filter, PCV valve, fuel filter, plugs, wires, clean your MAF sensor (very carefully with Qtip and alcohol), remove and thoroughly clean your TB.

If unknown mileage, or over 50-60k, replace the O2 sensor with an AC Delco sensor. Consider having your trans flushed.

Don't feel bad. My 1992 sse is perfectly tuned but I only get 14.5mpg city driving and I drive like an old man. I do however get 24mpg highway. My car has not thrown any codes either. If you want great gas mileage you'll have to get a 4 cylinder car.

another good thing to check for is to make sure your not runnin around town with the performance shift button, the car will shift at higher rpm'* and take down more gas

normal shift button for normal driving(best mpg) and performance shift for racing or havin a little fun!

Not true in all cases. Performance shift will only alter your shift points if you're at WOT. If you are driving normally, it'll do absolutely nothing for you.

Check your catalytic converter. It may be getting clogged. Among alot of other possibilities.