An useless but fun observation: you can do the litres/100km division and arrive at units of area. A family car might have a fuel consumption of 0.05 mm^2.
This is the cross-section of the imaginary pipe of fuel along the highway that a car with no fuel tank would have to pick up as it travels.
Actually, what you're arriving at is energy use per unit distance. The unit of area result you're getting (correcting from original content) is a misuse of units: "liters/100km" is more properly "liters of fuel/100km", where liters-of-fuel is a shorthand for quantity (that is, mass) of fuel used per 100km -- it's more convenient to measure fluid volume than mass (though you'll find that aircraft will measure fuel by weight). And that is still a shorthand for "energy contained per unit of fuel over distance travelled". That reduces to energy/distance, or: ( kg × m ) / s^2. That doesn't have a unit applied to it, though it works out to mass times acceleration.
So, as noted, "Liters of petrol" (or diesel) are actually a shorthand for "energy contained in a liter of fuel". There's an imperial unit for this, the barrel of oil equivalent, "barreloil" in GNU Units. This is equivalent to:
• 6.12 billion Joules
• 1.70 MWh
• 5.8 million BTU
• 0.000107 Hiroshima bombs
• 0.21 tons of coal
• 3.82e+28 electron volts
• 1.46 million kilocalories (food calories)
So, a 30 MPG car (7.8 liters/100km) is burning 0.049 barrels of oil per 100km, or roughly 83.4 kWh/100km.
I happened to catch a recent video Robert Llewellyn posted to G+ on the VW e-Golf, an all-electric version of the Golf (previously the Rabbit in the US)[1]. He mentioned getting approximately 15 kWh/100km driving performance, with the manufacturer claiming efficiency as high as 12 kWh/100km. This means that, overall, the electric vehicle is a lot more efficient than a typical internal-combustion vehicle. Checking on Edmunds.com[2], the 2014 Golf Diesel 2.0L 6 speed manual is rated for 30/42 mpg -- the eGolf uses nearly 7x less energy (6.95x) on the road. Mind that generating and transmission losses run around 30%, so the net efficiency gain is only 4.87x, but that's still a lot more efficient than an oil-powered vehicle.
The eGolf does offer far less range than the oil-powered version, however, as a result of the much lower energy storage density offered by batteries over liquid fuels.
This is the cross-section of the imaginary pipe of fuel along the highway that a car with no fuel tank would have to pick up as it travels.