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· NCOR
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11,237 Posts
10.4 driving conservatively with those mods and 245's...ouch. I get 12-13 with 33's and 3.73 driving like a teenager with very similiar mods. I would suggest changing your fuel filter if you haven't done it in the last 15,000 miles, also plugs that have to big a gap could give you incomplete combustion resulting in bad gas mileage. So in other words if you running $2 copper plugs just replace them with new copper plugs(gapped correctly of course). There are others but those are my immediate thoughts.
 

· NCOR
Joined
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11,237 Posts
350 vortec fuel filter should be in the driver side framerail under the driver seat. I just did mine this weekend. Not to hard, just be prepared for a 1/2 of gas to spill. The plugs should be changed too...they need them after that long. The fuel filter just needs a socket set to remove the plastic peice (slide back off of it, do not try to pry it forward) and two cresent wrenches to unscrew the lines, took me 40 minutes having never done it before. Mine was $7 at Autozone.
 

· NCOR
Joined
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11,237 Posts
If its a vortec removing the cats could potentially hurt mileage. Backpressure is need and I realize you have 40 series, but the combination of no readout from rear O2's and less backpressure could be loosing you a lot torque...which is bad for city driving.
 
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