Why your car is using more petrol than it should

It’s one of the universal complaints of car ownership that your car is using more fuel than it should, but there’s more to this observation than the occasional joke or complaint at the next braai.

The fact of the matter is that cars will almost always use more fuel than advertised, and this has to do with the figures and calculations that automakers use when testing their new products.

In South Africa, fuel consumption is measured in litres per 100 kilometres (L/100km), which is based on the Worldwide Harmonized Light Vehicle Test Procedure (WLTP).

The WLTP was introduced in 2018 to replace the older New European Driving Cycle (NEDC) standard, and measures everything from fuel consumption to pollutants, CO2 emissions, and battery range for electric and hybrid vehicles.

It’s also worth noting that China uses a different standard, known as the China Light Duty Vehicle Test Cycle (CLTC), which tends to be more optimistic than the WLTP rating.

The WLTP test is designed to better represent real-world driving conditions than older models, and uses a rolling road to capture data about a new car while simulating various scenarios.

Before a vehicle is tested, things like the tyre pressure are set to the manufacturer’s exact specifications, and the air conditioning, heater, and headlights are all turned off.

The test itself consists of four phases – Low, Medium, High, and Extra High – which measure the car’s performance at different speeds, including stops, acceleration, and deceleration.

The Low phase simulates urban conditions with constant stop-starts and average speeds around 18km/h, and the Medium phase mimics suburban driving at speeds ranging from 40km/h to 70km/h.

“High” refers to rural driving conditions at speeds between 55km/h and 100km/h, and Extra-High is the setting used to measure highway driving from around 95km/h to 130km/h.

All of these phases are measured in 30-minute cycles, after which the manufacturer calculates three main fuel consumption figures – an urban cycle, an extra-urban cycle, and a combined cycle.

The urban cycle is what it sounds like – a measure of how much fuel the car uses while navigating around town.

This figure typically represents the vehicle’s consumption at its worst, as a dense metropolitan setting means traffic lights, stop streets, and congestion.

The extra-urban cycle, meanwhile, reflects the car’s usage on highways and situations where it’s possible to maintain a fairly high and consistent speed.

In other words, this figure usually represents a car at its most efficient.

These two figures are then used to calculate the model’s combined fuel consumption, which is a weighted average of the two.

Since most cars will spend the majority of their time in the city, the urban cycle is weighted more heavily than the extra-urban cycle in the calculation.

It is this combined fuel consumption figure that manufacturers use to advertise how fuel-efficient their latest models are.

The trouble is that, while the WLTP is more reflective of real-world conditions than previous standards, it can be misleading to consumers who aren’t aware of what these figures represent.

Why your fuel consumption is higher than advertised

Most people will achieve noticeably worse fuel consumption than advertised because of how, when, and where they drive their cars.

This is nothing new, of course, as driver input has always had a significant effect on how efficient or inefficient on a car’s performance.

Aggressive acceleration and frequent stops while racing between red lights will result in poor economy compared to more conservative driving.

However, this still doesn’t explain the disconnect many motorists experience when comparing their real-world consumption to what the companies claim.

The reality is that most households spend the vast majority of their driving time commuting, which means slow speeds, congestion, stop signs, red lights, and even cold temperatures in the mornings.

Remember that the combined fuel consumption – the advertised figure – is a combination of the urban cycle and the extra-urban cycle, which means that, in practice, it is optimistic compared to what most owners actually experience every day.

To give a practical example, I own a current-gen Hyundai i20 1.0T, which has a claimed fuel consumption of 6.5l/100km.

However, about 95% of my trips consist of driving to and from work, the shops, and the gym, and since all of these locations are relatively close to where I live, I rarely use the highway.

There’s also a school on the route I use to get to work, leading to a lot of traffic in the mornings.

As a result, my car’s actual fuel consumption is often closer to 7.0l or even 8.0l/100km over a normal week.

On the other hand, I recently refuelled and reset the meter before embarking on a long trip to visit a friend, driving almost exclusively on the freeway with no traffic.

Doing this, my consumption averaged out to an impressive 4.0l/100km.

During months where I had a more even split of city and highway driving, my average was 6.3l/100km, which is pretty close to the claimed figure of 6.5l/100km.

All of this is to say that, if you are frequently using more fuel than you think you should be, it’s worth reflecting on how and where you drive, because if you’re spending 95% of your time driving in Cape Town’s rush-hour traffic, you’ll almost certainly be averaging your car’s urban cycle rather than its advertised combined cycle.

Even then, it’s worth remembering that the WLTP test is carried out under ideal conditions, and that real-world factors, such as tyre pressure and using the air conditioning, will negatively impact your economy.