Like most substances, gasoline expands and contracts with temperature changes. 60 degrees Fahrenheit is the U.S. government standard for fuel measurement. The temperature in a gas station's underground storage tanks affects the amount of energy provided by a volume of fuel.
When fuel expands, it becomes less dense and delivers less energy per gallon. Gasoline expands 1 percent for every 15 degrees Fahrenheit. Diesel expands 0.6 percent for every 15 degrees.
Gas station storage tanks are well insulated. Storage temperature stays about the same as when it was delivered by the tanker truck. Fuel delivered in warm climates could be significantly warmer than 60 degrees.
Mileage is measured accurately if fuel is 60 degrees when pumped. Even if you drive in hot weather, the fuel will expand in your tank and the loss of density is made up by the increased volume.
Most stations can provide an automated gauging meter report that indicates the storage temperature of their fuel. Underground storage means the temperature stays consistent day and night.
Temperature compensating pumps are not used in the continental United States but are common in Canada. These pumps adjust the amount of fuel delivered to compensate for density changes.
If gasoline is stored at 90 degrees in the underground tanks, 2 percent of the fuel's energy is lost at the pump. In this case, a car that should get 30 miles per gallon will get 29.4 mpg instead.
