I have new 2011 Prius IV. Two days ago I drove until guage was flashing for 10-20 miles, and the trip display "miles to gas station" was showing 10. I pull into station and fill tank, even with extra squirt after first shutoff. Pump reads 9.9 gallons! Does this mean I have 2 gals left? The manual says tank capacity is 11.9 gals. Should I risk going another 60 miles after guage says 10 miles to go for station? How can I estimate how much usable fuel I have left in a low tank?