Actually, you don't need to empty your gas tank. The most accurate method of calculating fuel efficiency may be done regardless of how much fuel you have left in your tank:
1. Fill up your tank and reset your trip odometer.
2. At any time, refill your tank (can be done at a 1/4 tank, 1/2 tank, or any level). At the same time, write down your mileage from your trip odometer (then reset for next time) and the amount in gallons that you just put in while refilling your tank. (You must FILL your tank both the first time [step 1] and this time [step 2])
3. Calculate your mileage by dividing your total miles from the trip odometer reading by the number of gallons you put into your tank. Use a calculator and be accurate in your figures.
For example, assume that your trip odometer reads 312 miles and you put in 13.46 gallons of gas. The calculation would be as follows: 312 / 13.46 = 23.179 which is rounded to 23.2 miles per gallon. Keep a calculator in your car for quick reference and a small notebook that you can track your mileage and date. Then, if your mileage decreases drastically, you will know to check for a problem such as a bad O2 sensor. Have fun.
The problem with a fuel efficiency guage is that it takes into consideration your driving habits and a pre set value for baseline data (what the car is SUPPOSED to get). It then factors in actual reads from you fuel tank--but it is not 100% accurate. If you were to check it against the actual (as determined by the method above) you would see a descrepency. They are still fun though