Only 12-30% of the energy put into a conventional car is used to move the car down the road, according to the US Department of Energy (DOE). The rest of the energy is lost to engine inefficiencies or used to power accessories.
Source: US Department of Energy and US Environmental Protection Agency, Fuel Economy Guide website.
The amount of energy loss varies depending on the type of driving: city, highway, or combined city and highway. The engine losses, such as exhaust heat and pumping, are higher for city driving than for highway driving.
There are no idle losses in highway driving, but losses due to wind resistance and rolling resistance are higher for highway driving than city driving.