Fuel Test

The real cost of fuel

Ever wondered what a trip to the shops costs? Worried how much money you burn in traffic? And do you know your car's real fuel economy figures?

You might know your own car’s fuel efficiency figures. You will have noticed that one figure told you the efficiency in litres per 100km (l/100km) on an urban cycle, another l/100km on an extra-urban cycle (whatever that means) and a third telling you your combined l/100km.

But what do those figures actually tell you? Those figures are conducted indoors on a rolling road within defined temperatures and controlled speeds. It’s all very scientific but it doesn’t say much about your real-world fuel use.

So, armed with a sensible supermini* and an electronic device that reads data from the engine’s fuel management system, we set our own real world tests. We filled up on fuel and asked the gadget to tell us how much each of our ordinary journeys cost.

Real-world scenarios

1) A round-trip to the supermarket
If you don’t drive to and from work then, chances are, the trip to the supermarket is the most regular journey you take. We drove a round-trip to a supermarket 4km away, a total of 8km on relatively flat, urban roads. Fuel usage was a surprisingly modest 0.2 litres - that's around S$ 0.35 at current prices.

2) Shopping around for fuel
Does it really make sense to shop around for fuel? Or should you just fill up whenever you get the chance? Our trip from one fuel station to another was 3.2km. Fuel usage was 0.087 litres, so a four-mile round-trip would use 0.174 litres - around S$ 0.31.

So, in other words, it cost us more than a sixth of a litre to get to a station that sold cheaper fuel. Then you have to factor in the time it takes you to drive out of your way. Unless you know a station that sells fuel at a price that’s considerably cheaper than average, it doesn’t often make very much economic sense.

3) Visiting a friend
We popped round to visit a friend who lives in the countryside. It’s an opportunity to drive on twisty roads, a 10.3km round-trip from the by-pass that takes in hills, bends and a few kilometres at speed.

You don’t get to use the throttle much, living in the city. So a country road can be a good excuse to drive a little faster.

We drove the same journey twice. First we drove it impatiently, getting high revs before changing up, braking at the last moment and driving purposefully.

Next then did the same drive but frugally, changing up as soon as possible, trying not to go beyond 2000rpm at any point, coasting downhill and anticipating the need to brake.

The difference was quite significant: Fuel usage was 0.21 litres, driven with economy in mind (that's S$ 0.38) but 0.33 litres when driving without heed to economy (S$ 0.59).

4) Idling at the drive-in
People in a hurry like to get their food quickly. But how much does it cost to queue at a drive-in restaurant? We tested our car on idle for five minutes. Fuel usage was 0.015 litres - that's S$ 0.03.

But beware of air-conditioning. It is said that air-con will cut your fuel economy when driving by 10 per cent. But when you’re idling it’s considerably more than that.

Fuel usage with air-con on was 0.028 litres (S$ 0.05), nearly double. So the lesson is: windows down, air-con off when idling or in heavy traffic!

How do you test your fuel economy?
If your car doesn’t have a fuel economy gauge, or you don’t have access to the kind of gizmo we used for this test, don’t worry – a little bit of simple maths is all you need to work out your fuel consumption.

Fill up your tank to the brim next time you need fuel. Set your trip meter to zero (it’s on your dashboard, usually next to the odometer). Next time you fill up, note down your trip computer reading and exactly how much fuel you needed to fill the tank.

Your average fuel economy in l/100km is found by dividing the fuel used by the distance driven and multiplying by 100. So if you’ve used 54 litres of fuel and driven 663 km, you divide 54 by 663 and times by 100 to get 8.145 l/100km.

* Notes on our test
Our test was conducted on real-road conditions using a 2008 Mazda 2 TS2 with a petrol engine