Under favorable conditions, a Chevrolet Volt can achieve 8 km per kwh. In urban driving, a Saab 9/11, a car of similar dimensions and performance to the Volt, achieves about 8 km per liter of gas. Since a liter of gas has an energy content of 9 kwh, the Volt is, apparently, nine times as efficient as the Saab. However, most electric power is generated thermally from coal or natural gas with an efficiency, after taking account of line losses and losses in battery storage, of no more than 50%. So the Volt has an advantage in efficiency over the Saab of at best 450%.
But the Volt is several hundred kilograms heavier than the Saab, the difference representing a quantity of embodied energy: energy consumed in mining and refining of the lithium in the Volt's battery, and in the additional steel, copper and other materials required to build the Volt's more complex power train.
Furthermore, the Saab 9/11 is far from being the world's most efficient internal-combustion-engine-powered automobile. Of similar size to the Volt, but with better styling, is the Chevrolet's Malibu hybrid, which achieves around 20 km per mile in urban driving, which means that carbon emissions per km by the Malibu are only about 80% higher than those of the Volt. That is when the Volt is operating under favorable conditions.
However, the Volt does not operate with anything like maximum efficiency under unfavorable conditions. If driven with some assertiveness, like, for example, a Saab yielding 8 km per liter, then the efficiency of the Volt drops by at least a quarter.
In addition, the Volt achieves high efficiency only when other loads on the battery are at a minimum. But when the outside air temperature is at the freezing point, the windshield wipers, are on, the rear-window defroster is on, the fan is at full blast to keep the windshield mist free, the headlights are on, as are the heated seats and steering wheel, then the efficiency of the Volt takes a further dip of at least 50%. Then the Volt may well be less efficient than the Malibu hybrid and perhaps many other efficient ICE-powered cars that use waste heat from the engine to heat the cabin and keep the windows clear in cold weather.