Fuel economy describes how much fuel your vehicle uses in relation to the distance travelled. It is given by manufacturers in a standard form for urban, extra urban and combined cycle driving which is determined by a standardised test, described here. While this test doesn’t resemble everyday driving, and therefore tends to be more economical than the figures drivers can realistically obtain, it does allow the performance of multiple vehicles to be compared against one another relatively.
Calculating a realistic fuel economy figure
The two ways of reporting fuel economy are in litres per 100km or kilometres per litre.
Litres per 100km (l/100km)
You will need the number of litres it took to fill your tank since the last time you filled it, and the number of kilometres you traveled in between fills.
The calculation is: litres / distance * 100 = l/100km.
For example: 57 litres / 635 km * 100 = 8.98l/100km.
If you know the price of fuel, then you can simply multiply the price per litre by the result and that gives you your cost per 100km. E.g. if fuel is $2, then 8.98l/100km means that it takes $17.96 of fuel to travel 100km, or around $0.18 per kilometre, not including your other costs like wear and tear.
Kilometres per litre
You will need the number of kilometres traveled between filling up your vehicle and the number of litres it took to fill the tank.
The calculation is: kilometres / litres = km/l
For example: 635 km / 57 litres = 11.14km/l
Note that this figure is quite different from the litres/100km figure.
Choosing a more fuel efficient vehicle
When looking for the best fuel efficiency you want to have a lower l/100km figure, or a higher km/litre figure. E.g. 4l/100km is more economical than 7l/100km, and 15km/litre is more economical than 10km/litre.
Bear in mind that individual driving styles play a huge part in the actual fuel economy achieved by a driver. Check out our guide here for simple techniques you can use to save up to 30% off your fuel bill.
Accuracy in the calculations
The calculation will become more accurate the more times you do it as it will average out specific events that negatively or positively affect your fuel consumption over shorter distances.
Keep a note of your fuel usage in a notebook in your glovebox and fill in the details every time you fill up. It can help to zero your trip counter. If you run your vehicle until the fuel light first comes on and then fill it up straight away you will get better fuel economy overall because your vehicle weighs less when the fuel is nearly empty. But, if you usually fill up once you get to a quarter tank, then you should use that point to get a more realistic idea over time.
Different fuel pumps can be calibrated slightly differently so if you want to be really consistent, use the same pump at the same petrol station. Atmospheric temperature and pressure can slightly affect the fuel, too.
You should be aware that the very act of knowing that you are measuring your fuel economy is likely to make you drive more economically!