Part of successfully managing a fleet’s expenses relies heavily on how to accurately calculate your fuel consumption. Inaccurate calculations can lead to incorrect budget predictions, the wrong vehicles being purchased, route inefficiency and a number of other problems that will negatively affect the success of your fleet.
So how is fuel consumption calculated?
Countries that use the metric system, such as South Africa, calculate fuel consumption in terms of the number of litres used over 100 kilometres (litres/100km). The formula looks like this:
(Total litres filled up Kilometers travelled) 100 Fuel consumption
The catch, as every fleet manager knows, is that no 100 kilometres travelled are ever the same, which is why it’s so important to do research on the vehicles you intend to put onto your fleet, as well as the driving conditions most commonly experienced.
Can manufacturer figures be trusted?
Manufacturer consumption figures rarely reflect real-world situations. It’s impossible for manufacturers to test their vehicles under all potential driving conditions around the world, so they opt for clean and consistent instead. Most manufacturers will conduct their fuel consumption tests under fairly ideal conditions and they’ll consistently test under the same conditions. That’s why published fuel consumption figures tend to be a lot more optimistic than true consumption figures.
But these figures are still useful: they can be used as a point of reference to compare manufacturers/vehicles to one another. Because they’re all tested under similar conditions, the manufacturer figures give a snapshot of how vehicles stack up against one another. It’s a good starting point when you’re trying to decide which vehicles will best suit your fleet. As a rule of thumb, the vehicles with the best-published consumption figures will also deliver the best fuel consumption in real life - the actual figures, however, won’t be the same as those published by the manufacturer.
So how do I get a real-life consumption estimate?
If you’re still in the buying phase, the best way to get accurate consumption figures is by speaking to fleet other fleet managers who have experience of the vehicles on your shortlist. Research through networking, as well as online reviews, will give you a feel for which vehicles are running efficiently and which are not living up to their promise.
If you need to get an accurate consumption figure for the vehicles currently on your fleet, the best way is old school. By keeping accurate vehicle logs, you’ll have the exact number of litres used by every vehicle in your fleet. Use this information, combined with the precise number of kilometres each vehicle has travelled to calculate the fuel consumption of the vehicle to date (using the formula mentioned earlier).
Most modern vehicles are also equipped with onboard computers that will display the instant, and average, fuel consumption. These figures are generally reliable, but remember that the computer continuously updates the information. For example, vehicle A might be extremely efficient, but if it’s just completed a 500 km journey during which it was driven very hard (and therefore consumed much more fuel than normal), the onboard computer will factor this into its overall equation.
If you’re depending on the vehicle's onboard computer to determine your fleet’s overall fuel consumption, it’s important to know its journey ‘history’, so that you take readings after tough as well as smooth journeys - that way you’ll get a more accurate picture. Another handy exercise is to work out the fuel consumption according to the formula above - compare it to the vehicle’s reading - and see how closely they match up.
Accurately calculating fuel consumption is easy if you keep an accurate log, and in most cases, you’ll be able to depend on the vehicle’s onboard computer. The difficult part is identifying (and predicting) all the factors that negatively affect fuel consumption. Route optimisation, for example, is crucial in keeping fuel consumption to a minimum, whilst fuel fraud has a nasty way of catching fleet managers off guard.
Like most things fleet-related, fuel consumption is influenced by many external factors. Although there’s a straightforward formula to calculate a vehicle’s consumption, there is no guarantee that the results will always be the same.