The only way you could use a partial tank theory to accurately calculate mileage, is if you started with a bone dry tank, put in 4 gallons, drove it till it was bone dry again, then you could calculate mileage for those 4 gallons, using odometer readings from the empty to empty cycle. Problem with that is, you have to run out of gas all the time to be accurate.
So the best way is to use full to full tanks. For those with hard money issues, don't let the tank get less than say half full. Keep it full, rather than run around on empty. Its easier to put 2-3 gallons in it to keep it full and easier on your pocket book, plus that way in the case of an emergency, you HAVE plenty of gas when you need it.
Plus you can accurately track your Mileage that way.
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