Source: US Department of Energy.
Cars tend to by inefficient in their use of energy since only a marginal amount of the energy they consume 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 technological efforts 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 size.