Average fuel consumption of a car

How do you work out the average fuel consumption of a car? Newer cars already have a built-in on-board computer that calculates the average consumption for you. If you don’t have such a computer, or for some reason you don’t trust it, you can work out the average consumption yourself.

You will need two things: to measure the distance you have travelled and how much fuel you have used in that time. The easiest way is to fill the tank right up and, from that moment on, start counting the kilometres you drive. After a while, drive to a petrol station and fill up again. If you have just put in, say, 40 litres of fuel, it means that since the last fill-up you have used 40 litres of petrol or diesel. Now look at how many kilometres you have driven since the last fill-up. Say it is 600 kilometres. Now, if you divide

$$\frac{600\text{km}}{40\text{l}}=15\text{km}/\text{l}$$

You travelled 15 kilometres on one litre of fuel. However, it is usual to give consumption per hundred kilometres. You find it by calculating

$$\frac{100\text{km}}{15\text{km}/\text{l}}=6.666\text{l}$$

We get a consumption of roughly six and two thirds of a litre (about 6.7 litres) per hundred kilometres. You arrive at the same consumption by another method, namely

$$\frac{40\text{l}}{600\text{km}}=0.0666\text{l}/\text{km}$$

So for each kilometre driven you use 0.0666 litres of fuel. If we multiply this value by a hundred, we again get the consumption per hundred kilometres:

$$\frac{40\text{l}}{600\text{km}}\cdot 100\text{km}=6.666\text{l}$$