A good approach is to start from real consumption data for comparable cars. It might be impossible to drive a Ferrari to consume only 10liters /100 km (23mpg), nevertheless it is a nice round figure for starters:
The Roadster stores 53 kWh and consumes about 240 Wh/m (150 Wh/km or 15 kWh/100 km, normal driving, summer).
A medium sized (US) car tank holds perhaps 12 USgal (45 liters) and consumes around 23 mpg (10 liters/100km).
Take 10 kWh per liter of gasoline, or 38 kWh/USgal.
The petrol driven car stores therefore 7 times more energy than the electric roadster, but consumes also nearly 7 times more energy per mile or km.
The Roadster stores 53 kWh and consumes about 240 Wh/m (150 Wh/km or 15 kWh/100 km, normal driving, summer).
A medium sized (US) car tank holds perhaps 12 USgal (45 liters) and consumes around 23 mpg (10 liters/100km).
Take 10 kWh per liter of gasoline, or 38 kWh/USgal.
The petrol driven car stores therefore 7 times more energy than the electric roadster, but consumes also nearly 7 times more energy per mile or km.