I agree with agnosto. Unless the MFD says the tank is doing over 45 mpg I don't try. In my experience, the MFD is often 1 to 1.5 mpg higher than my manual calculation. My last tank was 700.3 miles, using 15.8 gallons. The MFD read 45.5, but I got 44.3 actual. I usually fill up at the same place (Shell) and the same pump.

As others have noted, the TCH has about 3 gallons left when the low fuel light comes on, and about 2.5 gallons left when the needle rests on the E mark. I usually zero the B trip at E, and go around 40 miles or so and fill up. I don't want to use the last gallon up - it helps cool the fuel pump and is my emergency reserve.