Energy density and the cost, weight, and size of onboard energy storage are important characteristics of fuels for transportation. Fuels that require large, heavy, or expensive storage can reduce the space available to convey people and freight, weigh down a vehicle (making it operate less efficiently), or make it too costly to operate, even after taking account of cheaper fuels. Compared to gasoline and diesel, other options may have more energy per unit weight, but none have more energy per unit volume.
On an equivalent energy basis, motor gasoline (which contains up to 10% ethanol) was estimated to account for 99% of light-duty vehicle fuel consumption in 2012. Over half of the remaining 1% was from diesel; all other fuels combined for less than half of 1%. The widespread use of these fuels is largely explained by their energy density and ease of onboard storage, as no other fuels provide more energy within a given unit of volume.
The chart above compares energy densities (both per unit volume and per unit weight) for several transportation fuels that are available throughout the United States. The data points represent the energy content per unit volume or weight of the fuels themselves, not including the storage tanks or other equipment that the fuels require. For instance, compressed fuels require heavy storage tanks, while cooled fuels require equipment to maintain low temperatures.