For the average human body weight of 65kg, (Average female is 60kg average male is 70kg) a 1 km walk at say 6 km/hour burns about 50kcal of energy (kcal or kilo-calories are often confusingly shortened to just Calories). The 50kcal came from digestible foods, or the digestible chemical energy in the foods. 50kcal converts to approximately 0.06 kWh of energy.
If we assume that our average non gender specific person sticks to the recommended average Calories intake of 2250 Calories (kcals) per day (that is 2000 for women, 2500 for men), then depending on your eating habits this walk will have cost you in terms of food fuel between 20 pence and a £1, see the table below.
Taking a fairly small and efficient petrol car (20% efficient), it will require about 0.65 kWh of energy from the fuel source to drive it 1km at a reasonable speed and the cost of this using today's petrol price is also shown in the table.
|Spend on food per day (assuming a 2250kcal food intake per day)||Cost of 1km walk (50kcal or 0.06kWh)||Fuel cost to drive car 1km (0.65kWh)|
|£10||22 pence/km||8.6 pence/km|
Well this is a fairly crude calculation and is only an indicative approximation. It is also not very useful, given that most of us in the Western world over eat and need to burn off those excess calories anyway. It shows that driving uses more energy than walking but that the food energy is more expensive, we could have guessed that!
It is not the whole picture however, for full energy consumption the complete picture might also include the overall energy required to manufacture and transport the food and the gasoline. It is for example much less energy intensive to be a vegetarian or vegan, and if you like those exotic foods to be flown in by the supermarkets then this is another consideration in the energy calculation and ultimately in those all important CO2 emissions.