00 Monte 3.4 getting bad mpg..

My computer threw a p1406? i think thats what it was cant remember exactly now, but basically the code said the egr wasnt working, so i replaced it. Now the engine seems to run cooler idles at 650 where as before it was at 900-1100, and i think i may even have a little more pickup. Well ok so the problem is i am only getting 15-18 mpg in the city, going easy on the gas. Is that normal or low? Seems like when i first got it i was getting 25-28. What should i check if it is low. I know the air cleaner is fine, as well as the egr now, also new wire and plugs installed(bosh platnum+2s).

Has the oxygen sensor ever been replaced? Price of gas it might be worth the money to change it. Personally I dont like bosch plugs. Have had problems with them in the past on customer cars. I just stick with delco.

oxygen sensors havent ever been replaced, but the car isnt throwing a code so i really didnt want to throw 100 bucks out on a crapshoot.