Hello,
I have a 2005 Screw with stock tires, magnaflow SI/DO exhaust, K&N FIPK CAI, new fuel filter, 5.4 liter, 3V with 104,000 miles. I have not attempted to do the dreaded spark plug change yet. The passenger side exhaust manifold cracked and was replaced. I have ARE topper on back. I do light towing with my race boat (with trailer approx. 2500lbs). I just recently purchased a SCT tuner from Troyer and I am waiting for my custom tunes. The current canned tuned is the 87 octane tune. I am currently getting on a good day 12 1/2 to 13 MPG's when not towing. (330-350 miles per tank, when towing approx. 1 mpg less). I know I should be getting 15-17 mpg's. The engine does not run rough, so I am pretty sure the coils are still good. What else could be causing this poor gas mileage? Any suggestions?

 

I got the exact same setup as you- I get 10 miles per gallon. Sucks. I also have code p0420 low efficiency. Could I have bad injectors? Can that cause bad mpg's and a plugged up cat?

 

You had better get the injectors checked, quickly.

 

My '07 FX4 4x4 5.4L with Gotts Mod and SI/DO Magnaflow barely gets 13mpg's (hand calculated) with 75/25 city/highway driving. I'm about to get new plugs, oil/filter change, new fuel filter and tranny service. This may slightly improve my mpg's but I'm thinking 12-13 is to be expected with my truck.

 

maybe geographic location has something to do with it? colder climates require more fuel to keep the A/F ratios correct.
even with my short drive ratio (4.56:1 gears on 33" tires) i avg 14-15 mpg hand calculated between 50-65 mph not towing. routine maintenance and royal purple lube in engine, tranny, and axle may help me tho

 

Time to bite the bullet and do it. Also be aware of the 2005 injector issues.

 

change the fuel filter and see if that helps??