A gallon is the amount of liquid car fuel (gasoline, diesel, and sometimes vegetable oil) that fits into a standard US car fuel tank. A gallon of gasoline has about 31,000 calories (or 114000 BTUs).
You can see why people would be interested in how many calories are in one gallon of car fuel: a gallon of gasoline produces 29 pounds of CO2. That’s about the weight of the average American adult. CO2 is a long-lived greenhouse gas and the main driver of climate change. If we collected all the carbon dioxide produced by cars and hauled it to the curb like trash each week, it would weigh about 30 pounds per person.
The calories in gasoline come from hydrocarbon molecules that contain hydrogen and carbon atoms. When a gallon of gasoline burns, the molecules of the hydrocarbons break apart, and the energy they contain turns into heat. This heat releases the carbon dioxide, which floats away into the atmosphere.
To make gasoline, it takes a lot of fossil fuel (oil) to dig out, refine, and transport it to your local gas station. It also takes a lot of energy to operate the car engine and keep it running. The amount of energy required is often called a calorie-equivalent gallon, or GGE.
In physics, a calorie is the amount of heat it takes to raise one gram of water 1 degree Celsius. On a food label, the calorie is used to measure the energy content of a food. It is also known as a kilocalorie, or a large calorie. The average person consumes about 2400 kilocalories each day.