This constant represents the approximate amount of carbon dioxide (CO2) that is produced when you burn a gallon of diesel fuel which contains 20% biodiesel fuel. This fuel is referred to as B20.
"The US Energy Information Administration (EIA) estimated1 that U.S. gasoline and diesel fuel consumption for transportation in 2013 generated 1,095 million metric tons of CO2 from gasoline and 427 million metric tons of CO2 from diesel fuel respectively, for a total of 1,522 million metric tons of CO2.
This total was equivalent to 83% of total CO2 emissions by the U.S. transportation sector and 28% of total U.S. energy-related CO2 emissions."