In 1994, the US National Institute of Standards and Technology (NIST) defined "gasoline gallon equivalent (GGE) [as] 5.660 pounds of natural gas.
It can be measured by its volume in standard cubic feet (ft3) at atmospheric conditions, by its weight in pounds (lb), or by its energy content in joules (J), British thermal units (BTU), or kilowatt-hours (kW·h).
To start, only a fraction of the stored energy of a given fuel (measured in BTU or kW-hr) can be converted to useful work by the vehicle's engine.
This can be a manual adjustment using tools and test instruments or done automatically in computer-controlled fuel injected and multi-fuel vehicles.
Forced induction for an internal combustion engine using supercharger or turbocharger may also affect the optimum fuel–air mix and thermal efficiency.
Other dynamic losses can be caused by friction outside the motor/engine, including loads from the generator / alternator, power steering pump, A/C compressor, transmission, transfer case (if four-wheel-drive), differential(s) and universal joints, plus rolling resistance of the pneumatic tires.
Cars sold in the United States are advertised by their measured overall efficiency (fuel economy) in miles per gallon (mpg).
The MPGe metric was introduced in November 2010 by EPA in the Monroney label of the Nissan Leaf electric car and the Chevrolet Volt plug-in hybrid.
The ratings are based on EPA's formula, in which 33.7 kilowatt hours of electricity is equivalent to one gallon of gasoline (giving a heating value of 115,010 BTU/US gal), and the energy consumption of each vehicle during EPA's five standard drive cycle tests simulating varying driving conditions.
[5][6] All new cars and light-duty trucks sold in the U.S. are required to have this label showing the EPA's estimate of fuel economy of the vehicle.
(see chart above) A flex-fuel vehicle will experience about 76% of the fuel mileage MPG when using E85 (85% ethanol) products as compared to 100% gasoline.