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I had a 6th Generation Civic (1997 Si Coupe) for about a year, where I noticed that the fuel tank level would vary if the car was making heavy right or left turns (it would raise or lower depending on the turn and the duration of turn). Well, since I got the 7th Gen, that issue has been eliminated and I've noticed that everything seemed good. I've been averaging some 31 MPG with some heavy high-revving driving, so fuel economy isn't an issue... aside from the 26 and 27 MPGs I got in the last 2 weeks, which I account to a cheap gas (Marathon in Detroit) that was probably saturated with water or something.

Well, the main thing I noticed this weekend was my fuel tank level and the fact that it has been fluctuating back and forth. Last night, it was at the bottom red line, with the 'low fuel' light on. This morning, same thing... but as I drove some 20 km, the light went off and I noticed that the indicator went up two notches. WTF? This stayed up for about 10-20 minutes, and then dropped again a notch, but without activating the 'low-fuel' light. Driving another 20 km, eventually it dropped back to the red line and the light appeared. I didn't wasn't sure what was going on, so I decided to fill her up. And I managed to squeeze nearly 38 litres (~10.0 US gal) into the 50 litre tank (~13.2 gal). Why was it fluctuating, I dunno?

Well, theoretically, at the 27.4 mpg that I got this week, with about 3.2 gallons left, I could have driven another 87 miles (140 km) till the fuel tank was dry... but I don't feel like calling my buddies to help me get more fuel in my car. So then my other question, in addition to the fluctuation of fuel tank level, why does the indicator of 'low fuel' come on with about 24% fuel left in the tank? Isn't that a little too soon? I would have assumed that a gallon (3.785 litres) is the point where most indicators should come on, that even at a bad 25 mpg, you can still go another 25 miles (40.9 km) before being done.

 
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I had the same problem with my 2K2, but the real issue was my broken speedo. My dealer had to replace the entire cluster (not just the speedo head) and 1,400 miles later I have not experienced a problem with the fuel gauge again. I do know exactly what you are talking about as I witnessed the exact same fluctuation on perfectly level roads that would not lead to fuel shifting around within the cell.

Definitely take it in and get it fixed. The gauge clusters where on backorder when I needed one so that should tell you something about that component's suspect quality.