Completely understand--I was in shock when I calculated 46.
When I calculated that, I filled to first click, drove, filled again to first click. Between that time I drove 389 Mi. The gas station pump said I put ~8.5 gallons in my car. 389/8.5 = 45.7. Even if first click isn't full, and even if I can put 12 gallons in my 10.6 gallon tank, I would think that first click is always first click and 8.5 is always 8.5. You stated that you get 280 with first click, is this consistent? Maybe it's the weight of those 2 extra gallons you're hauling that kills your mileage (kidding).
Currently, on my trip meter (which I reset after putting gas in my car), I have 317 Miles. My gas gauge (which is, i agree, not at all accurate) says I have under 1/3 of a tank left. I'm predicting that this next tank will not be anywhere near as good as the last (45.7 MPG) as I have been a little heavier with the foot and have driven up an annoying number of hills lately. Although I should still squeeze out a nice MPG number, I don't expect (as my lifetime MPG shows) to get 46 on a regular basis. That would be nice.
Finally, I can't believe that the gauge that comes with the car is THAT far off. I've heard as high as 5 MPG (46-5 = 41, still good), but no higher. I can't believe it's off 10 MPG.
46 MPG does come with a price though, the car is a lot less fun (if zippy is as fun to you as it is to me) when driving for MPG. I think my lowest point was when my Mom (I'm 27, she's 55) turned to me and said "why are you driving so slow?"
So, where's my beer?
-P
https://www.fueleconomy.gov/mpg/MPG.do?action=calcMPG