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Putting aside the small obvious things like tire pressure, driving hard, etc etc... here's a couple major things that I know (from prior experience) that will decrease gas mileage severly.

1. Check your oil... see if you can smell a trace of gasoline in it. It's the indication of bad seals.

2. Incorrect Valve timing drops gas mileage... but bad timing would be noticeable most likely.

Both these things together happened to me at once on my last car. Dropped gas mileage from 28mpg to 13 mpg.

You could also get into the DIYs of checking injectors and fuel pressure. Fuel pressure you need a gauge and attachment to check. The injectors, you can half test just by putting a screwdriver to the top and your ear to the other end to make sure each is consistantly clicking the same speed. As far as correct pulse... you need a box that shows the pulse of the injectors.