Most drivers with a long commute are keen to keep fuel consumption down – but do many know the best way to do this?
Whether crawling between Dubai and Sharjah, or eating up the kilometres between Abu Dhabi and Dubai, there are ways to cut petrol costs, some of which go against what we might expect.
Few people have looked at the subject in greater detail than Prof Turlough Downes, of the School of Mathematics at Dublin City University.
In his day job, Prof Downes is an astrophysicist who studies how dust particles come together, over long periods of time, to form planets.
Air resistance is of central importance to this, and in the past few years Prof Downes has applied his knowledge to road vehicles, because aerodynamics is key in determining fuel consumption.
Finding the best speed
He has worked out what is roughly the “sweet spot” for cars on longer journeys in top gear.
“You will find most cars are designed so they’re travelling at their best between 80 to 90kph. For some cars it will be 80, for some, 90,” he says.
Above this speed, fuel use is heavily influenced by air resistance, because the increase in resistance is not proportional to speed, but to its square. This means that if a car accelerates from 80kph to 160kph, air resistance does not double, it quadruples.
When going from 80kph to 100kph, which is 25 per cent faster, wind resistance soars by about 50 per cent.
Prof Downes has calculated that, in part because of this, fuel consumption is about 28 per cent higher at 100kph than at 80kph.
“For some cars it might be 25 per cent, for some 35 per cent, but 28 per cent is the middle number,” he says. “It might differ from car to car [based on] what the size of the wing mirror is, what kind of tyres.”
Prof Downes tested his calculations on his previous car, a 2012 Seat Leon, and found that they tallied well with the real world.
Speed up further to 120kph, and fuel costs are 63 per cent higher than at 80kph, while at 140kph they are no less than 103 per cent higher. This means the car is burning more than twice as much fuel.
From Dubai to Abu Dhabi
What does this mean for UAE residents who have a long motorway commute, perhaps between Dubai and Abu Dhabi? Prof Downes has helpfully crunched the numbers for the UAE.
Based on a fuel price of Dh3.62 per litre, a journey length of 139km and a car of roughly average fuel consumption, he has calculated that at a constant speed of 80kph, the Dubai-Abu Dhabi journey takes 104.25 minutes and costs Dh30.19.
Press the accelerator to increase the speed to 100kph and the trip lasts 83.4 minutes but costs significantly more – Dh38.68.
Faster still, at 120kph, the journey time is 69.5 minutes and the cost is Dh49.06, or almost two-thirds higher than at 80kph.
Anyone driving at the top speed of 140kph would make the trip in 59.6 minutes, but their wallet or purse would be Dh61.33 lighter as a result. That’s more than double the cost at 80kph.
In his own driving, Prof Downes admits he is not a “hypermiler”, the name for people who try to squeeze every last mile out of a tank of fuel. He finds a reasonable compromise.
“I can only imagine what it’s like to travel at 80kph on a motorway,” he says. “But I do travel at 100kph, going at the same speed as lorries. I don’t feel I’m getting in people’s way too much.
“Time savings aren’t that great [from driving faster]. If you have a habit of going at 120kph the costs stack up month after month. I would rather spend the money on something else.”
Prof Downes now drives an electric car and he says similar figures apply when it comes to how costs increase with speed.
Other factors that force up fuel costs are driving in the wet and using a roof box. Prof Downes found out the effects of a roof box when he left one on his car for a trip across Ireland after returning from a holiday in France.
“I just didn’t have time to take off the roof box. I used about 50 per cent more fuel on that journey. I was just astonished, what it did. I thought it might increase it by 10 or 15 per cent, but it was more than that,” he says.
As well as creating drag itself, a roof box disrupts the aerodynamics of the car it is on, Prof Downes says, making its impact on air resistance greater.
“Keeping your tyres properly inflated is extremely important. If your tyres are a little bit softer, that will damage your fuel efficiency a lot,” he adds.
While, on longer journeys, weight tends not to have as dramatic an impact on fuel consumption as air resistance, it does have an effect, so Prof Downes advises drivers not to leave things in their car boot unless they are needed.
In heavy traffic, such as when stuck in a logjam between Sharjah and Dubai, the important thing is to try to maintain a constant speed.
“You don’t gain anything by going faster, because you will always be stuck behind the car in front of you,” Prof Downes says.
So if the car ahead rushes away as the traffic begins to move, the best strategy is to move forward slowly, as this way you may avoid having to use the brakes – and wasting the energy used in accelerating – as vehicles ahead grind to a halt again.
“Try to drive as if you’re not going to use your brakes,” he says. “Always try to anticipate when you’re going to slow down, so you keep your foot off the accelerator.
“Just cruise along as slowly as you can. If everybody goes as slowly as possible, you will get home faster … It’s very counterintuitive so it’s very difficult to get everybody to do it.”