The size of the gas tank does not matter when calculating gas mileage. Note the odometer reading with a full tank, drive until the tank is below 1/4, note the odometer reading and refill the tank, then divide the mileage differential by the gallons needed to refill the tank. For a more meaningful average MPG it's better to check mileage over several fill-ups. If you reset your trip odometer when you fill-up you can do a 'quick check' of the MPG each time you buy gas.

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Fred Hoelzle