Crazy Eddie,
Your right, mpg is not rocket science, its the simple ratio of Miles Per Gallon: divide miles driven by gallons consumed. The larger the sample the more accurate the results. A one tank tast will only be accurate if the the tank is filled to exactly the same level at the begining as at the end. You can't do that by putting in 10 gallons. The only practical way is to fill the tank both times until the fuel nozzle clicks off. There will be some variation with this, but averaging succesive tankfulls (using same pump if possible) will make the results more accurate.
I've been doing this for many years and many vehicles (my wife says I'm a bit anal about it)and have gotten to the point I can predict the fuel I will need to fill up based on the miles driven.
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