01 zr2 gets bad mpg's

hi i have a 2001 s10 zr2 that just got a few months back running normal size tire for the speedo ,not a lead foot and have a cold air intake. bigger exhaust and a bed cover on my truck .also had a full tune up besides the cap,rotor and wire .. but it has new trans fluid fresh oil change new plugs new air filter and everything.. the dealer said i should get between 15 and 18 mpg's in the city and about 275 miles per tank but i am lucky to get 200 per tank i am getting about 9 mpg and i can't figure out why. chevy told me they will look at it for about 75$ an hour so i said no. but i can't figure out how i am getting such crappy miles when the truck is in full running order any ideas would be helpful.. as for right now i am driving my chevy 1500 with the 5.3 and im getting better miles then my zr2 with a 4.3 in it so i know i am having a problem with the mileage if a big v8 is getting better then a little truck with a v6.. so any useful ideas would help out a- lot thank. also i am only running it in 2wd just to let everyone know.

The cold air intake and bigger exhaust may be the root of the problem.
Describe all mods.
What kind of air intake and what all done to exhaust and cat?
Did the mileage drop all at one time or after the mods?
MT

not sure what type of cold air intake believe its a k&n but doesn't look like it has the k&n air filter and the bigger exhaust is a 3" pipe no cat back to muffler flowmaster with dual 2" pipe out had a shop run the pcm on the truck and they said everything was fine with the mods...if anything they should give me better mileage more air in and more air out burns the gas better which make more power and should be better mpg's not worse...

Mods and more air in and out does not always work out good.
Sometimes it will kill gas mileage and sometimes power.

Did you lose mileage before or after the mileage drop?
Did it drop all at one time or creap up on you?

Also recheck the computer readings and check the fuel pressure real good.
MT

4.3's are pretty fussy with tune up parts. What brand of parts did you use, (plugs, cap, rotor, wires)? As MT mentioned, check fuel pressure and leakdown rate, then post your results. 15 to 18 around town, and at least 20 on the highway should be expected. Take a 10 mile ride down I-94 and check your odometer with the mile markers, should be within 5%. What ever amount the odometer is reading incorrect, your fuel mileage will be off that same amount.

well i don't know what the mileage was before the mods sense i bought the truck and never ran it i put all the mods on then started running the truck my odmeter is correct i am only getting about 9miles to the gallon the only thing i can figure is that i need to have my air fuel mixture recalibrated to my mods i believe i am running to rich becuase i have done many mods to cars before and they always increase mileage and power never decrease both the only reason i would be loseing power and mileage on both i would have to have a bad air to fuel ratio the more that i think about it... my fuel preasure is all running fine i am running within specs everything on the fuel system is brand new. complete engine was cleaned took oil pan off and engine was spotless no slug so that is not the problem so my only hope now besides the air fuel mixture would be to change my front and rear gear ratio to a better gear for mileage truck came factory with a 3.42 so i was thinking about maybe a 3.08but idk.