Buy a locking fuel cap?

 

This is a joke right?

 

 

 

 

You sure no one is stealing your fuel?
Apply a very thin strip of scotch tape spanning the gap on the fuel door that way you can see if its been tampered with, trail cam works too.

 

Okay, you don't have a clue so I suggest getting your head around your fuel delivery between the tank and the engine fuel rail. The tank has a small pressure line exiting the top of the tank, as seen from below, that is connected to the final fuel filter attached to the fuel pump. The fuel pump filter and attach point of the line will not be in view. That line is relatively short and is coupled to another line that goes straight to your fuel rail on the engine by means of a quick disconnect fitting. Find that line and quick disconnect and check them for security of attachment to each other, and follow it forward to get familiar with where it connects to your fuel rail near the front of your engine. That line is a flex line that joins the hard line going back to the disconnect noted above. Also, your fuel tank is plastic and may have a crack that's seeping fluid. Your nose will detect a leak before your eyes find it.

No tank leaks? With the engine running, inspect the line noted above from the fuel tank to the fuel rail for any leaks while under pressure from your energized fuel pump.

I believe your accounting of your lost MPG but that kind of loss is usually associated with an easily detected leak or, as mentioned by another poster, theft. A locking gas cap or other theft counter-measure seems in order if you have no leaks.

 

It would have to have a leak to use that much or somebody needed you gas no other reason.

 

I don't think this would relate as incorrect mixtures to those extremes should cause running issues/none running at all but I also had mine run at 3-4mpg. The difference for me was this was on LPG and not Petrol. My issue was one of the wires from the Lambda sensor was snapped. My best guess was my LPG system was unable to work out its air/fuel ratio so was over fueling a lot. Although I wouldn't dismiss it, I do doubt that if it were using 2x or more fuel than required, it would run without issue.

 

First thing for me - brim the tank, zero the trip and run till the fuel light comes on. Brim the tank again and note how much fuel it takes as well as how far you've gone.
This is an accurate measure of fuel mileage.