I filled up my 2005 Prius today and despite going 320 miles and having 3 blips on the screen, it only took about 4 gallons, less than half of what it usually takes. The mfd said about 55 mpg, an there is no way I could have gotten 80 mpg (it's what it calculated it out to be). Any advice?
Most likely what you're seeing is the effect of cold temperature on the rubber bladder in the gas tank, its quite common. Others recommend averaging several fill-ups together to get a more correct average value.
It's good that you are keeping track of miles traveled since last refueling. Ideally the MFD also shows mpg since last refueling, not mpg over a longer period of time. Since your data tells you that the car consumed 6 gallons of fuel since the last refueling, you could pump 6 gallons without concern.
