As fuel costs continue to soar, many people are looking to maximise what they have in the tank.
With petrol costing as much as €2.23 per litre, and diesel hitting €2.10 in some petrol stations, motorists are being dogged by fuel inflation.
Russia’s invasion of Ukraine, concerns over Russian oil exports to the EU and a sharp increase in transport activity worldwide since the Covid-19 pandemic have pushed prices to record levels.
In March, the Government cut the excise duty for diesel and petrol by 10 cent to 15 cent per litre amid pressure over the cost of living. A €100 a week subsidy for lorry drivers was also announced in March, after industry representatives raised concerns about the impact of fuel prices on the livelihoods of hauliers.
As the Government has come under increasing pressure to introduce further measures to tackle fuel inflation, motorists have been issued with advice by industry representatives as to how fuel spending can be reduced.
Suggestions such as avoiding unnecessary trips and removing heavy items from vehicles have been advised by the Society of the Irish Motor Industry.
According to one leading academic, driving slower is not the simple answer to cutting fuel costs. Instead, it is about finding the “sweet spot” on the speedometer.
Other factors — aerodynamics, weight and even weather — also come into play when trying to realise savings, says professor of mathematics and astrophysics Turlough Downes.
“This statement that slowing down will save you money on your fuel is not quite true,” he says. “Because, really, it changes from vehicle to vehicle. It is a matter of finding your own car’s sweet spot.”
Driving slower than the 120km/h motorway limit, or the 100km/h in force on national roads will theoretically use less fuel, but easing off the accelerator too much can have the opposite effect, the Dublin City University academic calculates.
“When you slow down too much, you need to be shifting into lower gears. [Then] the engine is doing more rpm [revolutions per minute, or the number of times the wheels fully turn over 60 seconds] than in higher gears,” he says.
“So, you’re burning fuel faster under those circumstances. Somewhere in between the extremes of not moving and going ridiculously fast there is an optimal speed for your car. ”
The Dublin-to-Cork motorway journey — a trip of around 250km — can serve as an example, where air resistance, also known as drag, is the dominant variable that should be considered.
If driving at significantly more than 80km/h, most of the extra fuel used above 80km/h is being burned to push air out of the way, since air resistance increases as a square — multiplying a number by itself — of speed.
Air resistance
“If you double your speed, your air resistance goes up by a factor of four, not a factor of two. If you triple your speed, your air resistance goes up by a factor of nine,” says Downes.
Increasing speed from 80km/h to 100km/h hikes resistance by around 50 per cent. Revving up to 120km/h, increases drag by 225 per cent. At 140km/h, as well as breaking the law, air resistance soars by 300 per cent.
Fuel costs rise accordingly, if not exactly in parallel. The “sweet spot” is when the car is in the highest gear — usually fifth or sixth — and the engine is not labouring, argues Downes. In most cars, this is usually between 80km/h and 90km/h.
“My previous car was a Seat Leon and it loved 80km/h in fifth gear. It was by far the most efficient speed,” he says. Many cars display the km/litre consumption, which can act as a guide.
On a 10km motorway stretch, driving at 80km/h takes seven and half minutes and costs roughly €1.20 for an average seven-year-old Irish car, assuming six litres per 100km, and fuel prices at €2 per litre.
Be as gentle as possible on the accelerator and gentle as possible on the brakes
At 100km/h, this takes six minutes and costs around €1.54. Rising to 120km/h, it takes five minutes and costs around €1.95, while at 140km/h it takes four minutes and 20 seconds and costs €2.44.
So, pushing down on the throttle from 80km/h to 120km/h shaves just two and half minutes from the stretch of journey, but costs 75 cent extra, a 63 per cent increase. Increasing speed to 140km/h costs €1.24 extra, a 103 per cent increase.
Travelling from Dublin to Cork at 80km/h takes around three hours and 15 minutes, costing about €31.20. Driving at 100km/h shaves 39 minutes off the arrival time, but will cost you €39.98.
At 120km/h, it saves another 26 minutes but fuel costs surge to €50.70, while over the speed limit at 140km/h knocks a further 18.5 minutes off the journey, but costs €63.38.
Of course, fuel costs vary substantially from vehicle to vehicle. A 2021 Hyundai Tucson, the most popular car in Ireland that year, will guzzle 6.7 litre per 100km.
So, driving the SUV to Cork at 80km/h should cost around €34.84. At 100km/h that rises to €44.64; €56.62 at 120km/h; while a limit-breaking 140km/h will cost €70.77, before any speeding fine.
Most popular car
In a 2014 Volkswagen Golf — the most popular car that year, with a fuel efficiency of 7.6l/100km — the same journey at 80km/h costs €39.52, €50.64 at 100km/h, €64.22 at 120km/h, and €80.28 at 140km/h.
Compare these examples with a 2021 Honda Jazz. Only 99 were sold in the State last year, despite it being a very fuel-efficient car, at 4.56l/100km. The Dublin-to-Cork trip costs €23.71 at 80km/h; €30.38 at 100km/h; and €38.53 at 120km/h.
“Whatever car you drive, travelling at 100km/h will cost about 28 per cent more than travelling at 80km/h; driving at 120km/h will cost you 63 per cent more, while travelling at 140km/h will cost you 103 per cent more,” says Downes.
“In the time versus money trade off, it never works to go faster.”
Air resistance is defined by the shape or aerodynamics of a vehicle, and is measured by coefficient of drag.
A Land Rover Defender, say, which is not built primarily for aerodynamics, has a drag coefficient of around 0.6. Downes' old Seat Leon comes in at around 0.33.
So, at speed (eg 120km/h), the Land Rover Defender will use anything up to twice the fuel to deal with air resistance than the Seat Leon. For the same reason, a car roof box has a startling impact.
“I left one on when I was travelling on a long journey to Mayo after a recent holiday in France, ” says Downes, “I thought it might push my fuel costs up by five or 10 per cent, but they went up by 50 per cent.”
Tyre friction
While weight does not impact air resistance, it does increase tyre friction on the road. More weight means the engine has to work harder, with costs rising proportionally.
Taking as an example a car that weighs a tonne (1,000 kilos), packing in a family of four (averaging 50 kilos each) increases the weight by 25 per cent and fuel use by the same amount. So, always empty out the boot.
But what about the difference between fuels — petrol and diesel?
Diesel, while more polluting, is the more cost-efficient, particularly over long distances. This is to do with the way a diesel engine burns the fuel more simply than petrol engines, which need spark plugs.
A typical petrol car will convert just 20 to 25 per cent of the fuel’s energy into motion. The rest of the energy is wasted, unless some is used for heating the vehicle. A diesel car raises that conversion to about 30 per cent.
Many cars have turbo chargers, which kick into action at a certain speed. These make fuel consumption more efficient by up to 25 per cent, but the higher speed needed to bring the charger into action itself costs.
In terms of so-called “premium” branded fuels, offered at forecourts for a higher price and claiming efficiencies, it is a simple calculation between the better mileage claimed and the higher price.
Then there is the experience of driving in towns and cities: “Forget about air resistance here, it is not important,” says Downes. “The thing to worry about is idling in traffic.”
‘Barely crawling’
Impatient types who accelerate and brake every time traffic moves are “wasting fuel to an unbelievable extent,” he adds. “It is far better just barely crawling along as slow as you can.”
Again using the rule of the square, accelerating to 20km/h instead of 10km/h sucks up four times as much energy, with most of it wasted: “Be as gentle as possible on the accelerator and gentle as possible on the brakes,” he advises.
Downes tries to drive “like I never have to use my brakes”, he says. “That is not quite as insane as it sounds, it just means you are always anticipating, trying to work out when you will have to slow down, then taking your foot off the accelerator.”
Cost savings can be 20 per cent or more, while the time loss is negligible, if anything at all.
Obviously, drivers should make sure their car is well-maintained, with the engine well tuned and the battery in good condition. While out of reach financially for many, electric cars are “of course the most efficient of all”.
“The characteristics of cars are so different, but it is really down to driving gently and trying to find the optimal speed for your car,” he adds.
“Just drive for a while, where you are not going faster than 80km/h, then do the same at 100km/h. Do it for a few days, because even weather has an impact, like on a windy day when fuel efficiencies are probably worse.
“It needs a variety of conditions. You’ll probably find that between 80km/h and 90km/h is the sweet spot in the car, and it is best not to go faster than that.”
This article was updated on June 8th, 2022