DCU Professor of mathematics explained, yesterday, how Eamon Ryan got it right today on slowing down to save fuel

— Driving at 120km/h consumes ~40% more energy than at 100km/h.
— Speed limits were cut in previous fuel crisis to save energy.

A Twitter thread posted yesterday by a Professor at the DCU School of Mathematical Sciences explains why transport, environment and energy Minister Eamon Ryan got it right today when he said driving at slower speeds will help save fuel.

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A reduction in speed works in two ways to reduce fuel usage for petrol and diesel cars — on higher speed roads, speeds over something like 80km/h exceed the optimal efficiency of most cars and, within urban areas, a lot of energy is wasted slowing down and speeding up again, especially but not only with hard acceleration and stopping.

That’s the reason why lower national speed limits were introduced in many countries when the 1970 oil crisis hit, and more recently some advocates have suggested it as a way to lower carbon emissions.

Professor of astrophysics and mathematics, Turlough Downes, said he posted the thread on Twitter because he thought maybe people didn’t know about it, or had forgotten about it, and he hoped it would be helpful.

Professor Downes wrote: “For those worried about #FuelPrices for cars: if you drive significant distances then you can *easily* reduce your fuel costs by around 40%. tl;dr speed kills fuel efficiency and driving slower makes a *big* difference .”

“Most cars get their best fuel efficiency between 50 km/hr and 80 km/hr. Above 80 km/hr the majority of your fuel is being used to push the air in front of your car out of the way. If you drive at 100 km/hr along a particular stretch of road then you will use about 50% more fuel than if you drive at 80 km/hr to cover that same stretch of road,” he said

“If you do motorway driving and suppose you have an average fuel efficiency car (say 8 litres/100 km) then a 10km stretch will cost you about €1.60 at today’s prices, if you drive at 80 km/hr. Drive that 10 km at 100 km/hr that same distance will cost you €2.40,” said Professor Downes

He said: “Now drive that 10 km stretch of motorway at 120 km/hr and it will cost you €3.52. Slowing from 120 km/hr to 100 km/hr will go a long way towards erasing the impact of the recent #FuelPriceHike on your wallet: it reduces your fuel costs by about 40% for the same distance.”

He added: “Lastly, if you don’t drive on main arterial roads but do a lot of heavy traffic driving then drive as slow as possible: no point in burning fuel to accelerate when you’ll just need to scrub that speed after a few metres.”

So, why the sharp reaction to Ryan’s comments?

IMAGE: Nobody is suggesting everybody should cycle, walk, use the bus or use car share all the time.

COMMENT & ANALYSIS: Campaigners or professionals who have been involved in suggesting anything slightly disruptive to the status quo on cars will likely be aware of a very common occurrence — suggestions are dismissed out of hand regardless of how much evidence are behind them, and often motorists think you’re out to take their car from them.

I don’t say this lighly: It’s like a car version of how it’s continuously claimed that US Government are going to take away everybody’s guns when such would be impossible on a large scale in the US without something close to civil war.

Some people might be annoyed with Eamon Ryan’s comments because they think that anything Ryan says feels like he’s lecturing them. But others on Twitter and elsewhere thought he was just wrong, and that was central to their problem. In fairness, many accepted it was explained.

Even if Ireland had the world’s best systems for walking, cycling, or public transport, there will be people who need to use cars at least some of the time. Very few advocates or professionals in this area think otherwise.

Last week we even had the head of the RSA say that a campaign promoting less car use would amount to shaming people — which is bonkers given that the car industry has huge budgets to advertise cars, doing things like convincing people they need SUVs for driving around towns and cities, which has caused a huge jump in carbon emissions compared to if people stuck with normal-sized cars.

In Ireland alone, the car industry spends tens of millions of euro per year on marketing and advertising. One estimate IrishCycle.com has seen suggests €25m per year in 2005, and that will have increased since then. Does anybody really think that has no effect? Did Irish people’s need for SUVs really increase that much in the last few years?

But when anybody suggests a cycle path, a pedestrianised street or a bus lane it’s often treated as a personal attack on every motorist. At least that’s how many see it.

In Ireland, this feeling seems to be held the strongest among people in rural areas. Literally nobody — or at least as close as literally as you can get on these issues — is suggesting motorists in rural areas should ditch their cars.

This is one of Minister Ryan’s recurring problems — he’s often derided online and in the media for something he didn’t say — the suggestion that a village could use a set of car share cars between them and sell their own cars. At the time, I was a bit shocked by what people said he was saying, so, I went back and watched the Virgin Media daytime TV interview where it made his comments.

At the time, Ocean FM Villages reported it as “Villages in rural Ireland with 300 families could voluntarily restrict themselves to 30 cars per village and share transport needs”. Forget about the facts such as he didn’t say that, or that a rural car share scheme was being tried at the time. Let’s just never mind that there are car-free and one-car households in towns and villages that would benefit from access to such a scheme rather than getting a second or third car which they only need some of the time.

To everybody who claims, “but he was saying that”: he wasn’t and I’ve heard similar things over and over again assumed when people talk about other sustainable transport plans and measures.

This is a classic issue that many campaigners, councillors and transport professionals have experienced — if you suggest that some people or people, in general, would benefit from an alternative option, then it’s claimed that you want everybody to abandon their cars and only use that alternate for every trip.

But the truth is set aside when it’s memeified.


  1. I’m about to do a long distance drive for the first time since 2018, in a 16 year old heavy petrol automatic car. Needless to say, I’m not enthused and I’m as frustrated as ever that so many parts of Ireland are infeasible to reach by using the train & bringing my bike.

    That said, Turlough’s post is about 30 years out of date. The Irish fleet is predominantly diesel, which has much longer gearing than petrol and is much more reliant on being in an efficiency sweetspot related to torque&gearing.

    Modern 6-8 speed diesel cars are not going to lose 50% effiicency going from 80km/h to 100km/h. Some will gain efficiency, most will be relatively as efficient, some will lose efficiency. While air resistance grows exponentially, this is counteracted at <100km/h speeds by gearing – and modern cars (even SUVs) are more aerodynamic than in the past.

    Being a professor of mathematics is irrelevant to seemingly not really understanding how modern vehicles work – scroll through https://www.google.com/search?q=fuel+consumption+speed+graph&newwindow=1&client=firefox-b-d&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjDpd2w2Lv2AhUZQkEAHX9xBoEQ_AUoAXoECAEQAw&biw=2560&bih=1315&dpr=1#imgrc=Sp4Q9n3vFsQwQM for many examples.

    What makes a much bigger difference than 'slowing down', is a) driving more gently (i.e. accelerating slower, decelerating slower, using engine braking) & b) making fewer unnecessary journeys using your vehicle.


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