So I own a 04 ford f150 5.4l lariat, the issue I’m having is that my truck shows I’m getting 11.8mpg throughout city and highway driving keep in mind I’m not towing anything either. Even when sitting in park my mpg will fluctuate and drop 3 or more then once I start driving it changes and almost settles but still goes down very fast but it’s not just that my fuel gauge is going down as that is too and just as fast. I don’t believe I’m burning any fuel, the previous owner of the truck removed the cats meaning 02 sensors aren’t there. I read that could cause mpg loss but not that badly. My father and I are thinking maybe the float on the fuel pump could be deteriorated causing an inaccurate reading but again not 100% sure. I have gotten all my codes read and nothing that is fuel related. Anyone else have this issue and could shine some light on this issue?

 

These trucks are computer controlled OBD2 systems. For the computer to be “happy”, it has to read the upstream & downstream O2 sensor readings. It does this of course to adjust timing & fuel trims in real time. When you remove the cats, the downstream O2s will send unhappy signals to the computer. When you remove both the cats & O2s the computer is unhappy that it can’t read any signals.

I would expect your mileage to be lower than “normal” with what you have.

 

You can only accurately determine fuel mileage by filling up at a gas pump to kick off, drive it a hundred or so miles, then doing another fill up to kick off just like the first, then divide your 128.7 miles you drove between fueling stops by the 9.035 gallons you put back in to see the 14.2446 miles per gallon result. 128.7 / 9.035 = 14.24460431654676 actually but in real life, I'd call it "14 and a quarter".

What a gas gauge shows is just a wild *** guess on it's part subject to a float bouncing on the waves in a choppy sea of gasoline or a hillside parking spot.