Car Advice

How to calculate fuel consumption the right way

One of the most important aspects of planning a trip into remote locations is to calculate your expected fuel usage, your vehicle’s range and how much extra fuel you’ll need to carry. Here’s how…

When travelling in remote areas you need to ensure you have enough supplies to last the distance, and the most important of these are water, food and, of course, fuel.

Calculating consumption and range

Calculating a vehicle’s fuel consumption is not difficult. Simply note down the distance travelled since the last top-up and then take a note of how much fuel it consumed to travel that distance, then divide the litres used by the kilometres travelled and multiply by 100 to calculate consumption in litres/100km.

For example, if your vehicle consumed 60-litres of fuel to cover 450km, divide 60 by 450 and then multiply by 100 and the result is a rounded fuel consumption figure of 13.3L/100km.

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Armed with a fuel consumption figure, you’ll now be able to calculate the touring range of your vehicle by dividing the vehicle’s fuel tank capacity by the fuel consumption and, again, multiplying by 100.

For example, if your vehicle has an 80-litre fuel tank and it uses 13.3L/100km (as in the example above), divide 80 by 13.3 and multiply by 100 and the result is a rounded range of 601.5km. With the potential for varying fuel consumption in normal on-road driving conditions, you’ll want to subtract about 50km from this result to come up with a ‘safe’ touring range, which in this case would therefore be 551.5km.

Once you know your vehicle’s touring range, you’ll be able to plan your trip by seeing if you can safely cover the distance between available fuel stops along the intended route, or whether you’ll need to carry additional fuel.

What are some of the variables?

Ascertaining your vehicle’s true average fuel consumption is best done over a period of time – the longer the better. I note my vehicle’s fuel consumption in a log book every time I fuel up, which over time gives me a more accurate idea of ‘average’ fuel consumption. This is important as there are many factors that can affect fuel consumption, including vehicle condition, tyre pressures, load, driving style, fitment of roof racks, whether or not you’re towing a trailer, and the terrain.

If a vehicle that usually averages 10.5L/100km, for example, suddenly starts to use 12L/100km without a change in driving conditions, then you’ll know there’s probably something wrong with it. Mechanical factors that can affect fuel consumption can be as simple as incorrectly inflated tyres or a clogged air filter through to something more complicated that might require a mechanic’s attention, such as blocked injectors or some other mechanical fault. In the case of some modern turbo-diesel engines, for example, it could be that the Diesel Particulate Filter (DPF) has performed a burn cycle, and normal fuel consumption will resume once the cycle complete.

Driving style can also impact fuel consumption; if you accelerate and brake hard you’ll use more fuel than if you try to maintain a relatively constant speed by looking ahead, predicting traffic flow and slowing gradually and accelerating smoothly as required.

The load you have on the vehicle, and how it’s packed, will also influence fuel consumption. A heavily laden vehicle will require more energy to accelerate than an unladen one and will therefore use more fuel. And a vehicle with bulky goods loaded high on a roof rack will not be as aerodynamically efficient as a vehicle with nothing on the roof, particularly at highway speeds. In fact, some studies have suggested that fuel consumption can increase by as much as 25 per cent with a loaded roof rack, but whether this is the case with your vehicle will depend on other factors including engine type, fuel type, transmission type and the vehicle’s original aerodynamic efficiency.

Towing a trailer can also have a big influence on fuel consumption; not only does your vehicle have to cope with a greater load while accelerating, it will also be less aerodynamically efficient with a trailer attached, especially a big one like a tandem-axle caravan. In addition, your vehicle will have to overcome the added effort required to pull the trailer’s tyres along the road. The heavier, the bigger and the more wheels the trailer has, the more effort will be required to move it and the greater the influence on fuel economy. Depending on vehicle, trailer size and driving conditions, towing fuel consumption can be more than 30 per cent greater than when not towing.

What role does terrain play?

The terrain you drive in will also have a huge impact on fuel economy. Low-range driving in soft sand will use a lot more fuel than cruising down the highway at 100km/h so, if you’re planning a desert trip, you’ll need to take this into account.

One way to calculate expected fuel consumption for a desert trip, for example, is to load up your vehicle as you’d pack it for your trip away and drive it on some nearby sand. If this isn’t a realistic option, ask people who have driven where you’re intending to travel, but bear in mind that some vehicles can be more fuel efficient in certain off-road conditions than others.

Weather conditions can also have unexpected consequences when it comes to fuel usage. On the road, driving into a headwind can result in increased fuel consumption, but on the flipside a tailwind can aid fuel economy. Off-road, if you’re in an area that’s prone to turning to mud in wet conditions, it could be a hard slog to get out if it rains instead of the anticipated ‘easy’ drive, in which case you’re going to use more fuel than you may have planned for. For this reason, you should always carry more fuel than you’ll need – in other words, build a safety margin onto your calculations.

Carrying extra fuel

If you don’t think your vehicle will have sufficient fuel range for a specific leg of a trip, you’ll need to figure out the best way to carry additional fuel. There are several options…

The obvious way to carry extra fuel is in jerry cans, but these can be heavy and unwieldy things, especially when full and if secured in a difficult-to-access spot on the vehicle.

It should be noted that jerry cans should not be stored inside the vehicle cabin with people, and they should be kept well away from potential ignition sources (like electrical items such as portable fridges) and should always be placed on the ground when filled – never fill them in the back of a wagon or ute. A dedicated jerry can holder at the back of the vehicle is one of the best solutions for carrying jerry cans or, as a last resort, one up on a roof rack.

A better solution for carrying extra fuel is to fit a long-range or auxiliary fuel tank to your vehicle. This arrangement offers several advantages, including no need to manually handle extra fuel and the fact the extra fuel will be stored down low in the vehicle where it won’t have an adverse effect on vehicle handling.

Keep an eye on it

By regularly monitoring fuel consumption throughout a trip you’ll have a better chance of spotting any irregularities, which could include a mechanical problem or even a leak.

A daily vehicle check should become second nature when travelling in remote areas. Ensure this includes a thorough examination of the vehicle’s fuel tank(s) and fuel lines, as well as the condition of the fuel filter. If you’re carrying jerry cans, make sure they’re secure and they haven’t copped any damage.


Dean Mellor

Dean Mellor