Paragraph T.2.1.2 (b) of the National Institute of Standards and Technology Handbook 44, “Devices Indicating in Inch-Pound Units,” says that:
The acceptance tolerance on normal and special tests shall be 1/2 in3 plus 1/2 in3 per indicated gallon and never less than 1 in3.
This means that the accepted inaccuracy of a gas pump is half a cubic inch (over half a US tablespoon) plus half a cubic inch for each gallon and never less than one cubic inch. When you buy ten gallons, the pump can over- or under-dispense five and a half cubic inches of gas and still be in compliance.
So how much is five and a half cubic inches of gas? About 0.024 US gallons, or over twenty times the accuracy implied by showing a thousandth of a gallon on the pump. On one gallon of gas or less, the allowed inaccuracy is 0.004 gallons, or four times the implied accuracy.
This is also the best case scenario: Paragraph (a) doubles the tolerances for machines between repair intervals. Heat can also play a significant role, as all industry tolerances assume 60Â°F — a gallon of gasoline at 100Â°F takes up almost 3% more space.
Next: Gas Station Soda Machines — Thirty-Two Ounces of Lies!
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