Convert from Btu to gallon [U.S.] of automotive gasoline

Unit Definition (Btu)
The Btu is a unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In America the British thermal unit is sometimes called the heat unit. In defining the Btu, it is necessary to specify the temperature of the water; thus there have been several definitions over the years. However, one Btu is equal to about 251.996 (small) calories, or 0.251 996 of the (kilo-)calories counted by dieters. Using the current definition of the calorie (the IT calorie), one Btu equals approximately 778.169 foot pounds, 1.055 056 kilojoules or 0.293 071 watt hour. The symbol BThU has also been used, especially in Britain.

Unit Definition (gallon [U.S.] of automotive gasoline)
U.S. liquid gallon is legally defined as 231 cubic inches, and is equal to (exactly) 3.785411784 litres (1 L = 10-3 m3) or about 0.13368 cubic feet. This is the most common definition of a gallon in the United States. The U.S. fluid ounce is defined as 1/128 of a U.S. gallon.