Source: US Department of Energy.
Typical Energy Use for a Car
Cars tend to by inefficient in their use of energy since only a marginal
amount of the energy consumed by a car is actually used to provide momentum
(around 15-20% depending on the type of vehicle).
What remains is lost in the engine, the power to wheels, parasitic
losses (e.g. alternator, pumps), the drivetrain and when the
vehicle is idle (e.g. at a traffic light or in congestion). For
engine losses, it is the thermal losses, such as for the
radiator, that are the most significant, while for power to
wheels losses wind and rolling resistance are the most
significant. Energy is also used to either heat or cool
the section of the vehicle occupied by passengers. There are thus
made, such a with hybrid cars, to lessen non-momentum energy consumption
such as using the braking system to recharge the car battery or
automatically shutting down the engine while the vehicle is idle.
Any improvement in fuel efficiency leads to substantial
reduction in the energy consumed by vehicles due to the fleet