Today's cars are designed for idiots. Fuel gauges are pessimistic in order to reduce the number of incidents where people run out of fuel. Temperature gauges are damped so that normal temperature fluctuations won't move the needle and send countless numbers of paranoid people to waste a dealer/repair shop's time.

Most cars will reach "E" and still have enough fuel left over to travel another 20-30 miles. There may be 1-2 gallons left in the tank after the fuel light or the needles hits "E".

To get a good idea of your mileage requires consistency. Fill up at the same pump if possible for at least two consecutive fillups. Try to keep your driving style and conditions the same. Reset the trip meter at every fillup. Take note of the gallons it takes to fill the tank, as well as the trip meter mileage. Do the math from there.