Another Fuel question

I purchased a 06' Prius at the beginning of the summer and use it as my daily driver. From the beginning, the car consistantly averaged 47-49 mpg over the last 12000 miles or so. I usually fill up when the tank guage hits the last 2 bars and it usually takes about 8 gallons or more to fill. 5 weeks ago I noticed that it only took 6.5 to 7 gallons on fill up and it's been that way ever since. This alone would not have bugged me, but now I only go about 50 to 70 miles before the full bar's gone; in the past I used to go well over a 100 miles. I also noticed that a tank lasted me over 400 miles, now I'm pulling in to fill up at 330 miles or so. My mileage has also dropped down to 44-46 mpg. Any suggestions?

 

That's probably it, thanks for the info.