35 ILCS 505/1.8

"Gallon" means, in addition to its ordinary meaning, its equivalent in a capacity of measurement of substance in a gaseous state. In the case of liquefied natural gas or propane used as motor fuel, "gallon" means a diesel gallon equivalent as defined by Section 1.8A of this Act. In the case of compressed natural gas used as motor fuel, "gallon" means a gasoline gallon equivalent as defined in Section 1.8B of this Act.