Would taxing cars by weight be the fairest future option?

The car taxation system is going to have to evolve to meet the electric revolution

Weight, as anyone who has ever shuffled on the scales at Unislim knows, is a curse. It deadens responses, reduces efficiency, and generally encourages sloth and waste. And that’s just me.

For a car, weight is even worse because it’s the beginning of a vicious cycle. A heavier car needs a bigger, more powerful engine (or electric motor) to shift it about. More power requires more cooling (for the engine, or for a big battery) and the cooling system itself adds weight. Then you need bigger brakes to slow all of that extra weight down, and guess what? Bigger brakes are heavier, too… And so it goes on.

Light weight, by contrast, is the beginning of a virtuous circle. A lighter car can use a smaller, more efficient engine, which needs less cooling, which requires smaller brakes, which also brings aerodynamic benefits, which makes it more efficient. The fact that smaller, lighter cars are often more fun to drive is, of course, entirely beside the point…

Excess weight could, above all else, prove to be a better target for taxation than carbon emissions have been. Almost as soon as the carbon-based motor taxation system was introduced, car makers began loopholing their way around it, with the most famous example being that a diesel-engined executive BMW 5 Series became cheaper to tax than a bargain-basement petrol Ford Fiesta. Tax systems are, above all, supposed to be equitable and that certainly wasn’t.


While shifting the tax burden more to those who can afford it – more luxury tends to add more weight, after all – a weight tax for cars might also be a fairer system for the electric car future. Currently, the Government is facing down a barrel of ever-declining motor tax and fuel duty revenue as the switch to electric motoring takes hold. Generally, the solution most often put forward is road pricing – making drivers pay for each individual trip that they take.

That, though, is potentially fraught with difficulty not least from a point of view of privacy – do you really want the Department of Transport knowing precisely where and when you're driving? – but also of geography. Given the paucity of public transport options outside the major urban centres in Ireland, is it really fair to ask rural communities to pony up for road pricing when they often have no viable alternative to driving? Equally, many may – rightly – grumble that the sorry state of many rural roads makes such payment untenable.

So, tax the the weight of the car instead? Not only would it be fairer than carbon taxation (and equally linked to consumption given that a higher weight means a less efficient vehicle), and more private than road pricing, it be a spur to car makers to develop lighter, simpler, more efficient vehicles.

Trim their carbon emissions

Sadly, it may not be that simple. Conor Molloy, from Authentic Energy Management Services, an Irish company that helps other companies trim their carbon emissions, reckons that a weight tax would be fine, but that a road pricing system will be needed to run in tandem with it.

“A weight tax is a tax on ownership, and it’s the Government sending a signal, which is ‘buy the lightest car you can.’ And that’s fine,” Molloy told The Irish Times. “When it comes to road pricing, that’s a different issue. As a rule, we drive an awful lot more every year than our European neighbours. We do around 16,000km a year, they do more like 10,000km, so we need to find levers to reduce that.

“If someone said we need to move into tax by weight, that would be fine by me, but you’ll need road pricing of some description because we won’t have that €3 billion coming in from excise duty on fuel, and someone’s got to pay for the upkeep of the roads that we’re driving all our shiny electric cars and electric bikes on.

“It took us 50 years to disperse our population, to places where you can be sure they wouldn’t be living if they couldn’t drive, and it might take us 50 years to right-size society so that people aren’t driving too much.”

Car makers are equally less than keen on the idea of a weight-based tax. One such already exists in France, where drivers have to pay an additional levy for every kilo over 1,400kg that their car weighs.

Francois Roudier, who manages communications for Peugeot in France, told The Irish Times: "There is already a close correlation between CO2 emissions and mass. If the mass tax targets heavy vehicles on the grounds that they consume more fuel and emit more CO2, then the tax is redundant because there is already the CO2 penalty. If it is a question of taxing a 'heavy' vehicle for externalities other than CO2 emissions – brake and tyre emissions, material consumption, congestion, no in-depth impact study has been carried out."

Roudier is also doubtful that using a weight tax to spur innovation – such as the development of lighter vehicles based around carbon-fibre and aluminium rather than steel – would be of much actual use. “Vehicles today are made up of 70 per cent by mass of metallic materials, 60 per cent of which is steel. Replacing it with aluminium, which is lighter, is counter-productive in terms of CO2 emissions because the carbon content of aluminium is higher than that of steel. Replacing it with composite materials, such as carbon-fibre, is counterproductive in terms of vehicle recyclability because steel is almost entirely recycled, unlike composites, which are not,” said Roudier.

Fiscal reliance

Even so, taxing weight might reduce the public appetite for – and car companies' fiscal reliance on – large, heavy SUVs. A study published by the International Energy Agency (IEA) found that if you take SUVs out of the equation, the oil consumption of cars across the globe fell by 10 per cent in 2020.

Some of that is down to increasing sales of electric cars, while some is down to improvements in fuel economy. Add SUVs back into the equation, and that improvement is cancelled out. According to the IEA, in the decade running up to 2020, global emissions from “conventional” cars fell by 350 megatonnes of CO2, but emissions from specifically SUVs rose by 500 megatonnes.

The simple maths of the problem can be seen by comparing two cars using the same mechanical package. Both the Volkswagen Passat estate and the Tiguan SUV use the same MQB chassis, the same electronics package, and the same 2.0-litre 150hp “Evo” TDI diesel engine.

Both cars have seats for five, and large boots (the Passat estate can swallow 650 litres, while the Tiguan manages 615 litres if you slide the back seats all the way forward). However, the Passat has official CO2 emissions of 127g/km, while the Tiguan – same engine, same basic structure, same DSG automatic gearbox – has emissions of 145g/km.

Consumer advocates Which? reckons that SUVs are far worse than estates when it comes to both fuel economy and emissions. “Our independent tests reveal that, on average, SUVs emit much more CO2 than conventional models. Our lab-measured tailpipe emissions tests, tank-to-wheel CO2, which are more stringent than the official emissions tests, show just how big the difference is” said Which? According to its measurements, Which? found that a medium-sized diesel SUV emits 171g/km on average, compared to 136g/km for a “medium car” or 155g/km for a “large car”. Both of those latter categories include luxury and high-performance models.

It’s the same story in terms of fuel economy, where Which? found that a medium-sized SUV could cost you more than €300 a year extra in fuel expenses, while even small “crossover” SUVs were around 7 per cent less fuel efficient than mechanically similar small hatchbacks. Given how quickly a CO2-based tax turned us all into diesel buyers, surely a weight-based tax could finally turn us away from hefty, profligate SUVs?

Extra flab

“My first reaction to this proposal is to leap around punching the air and whooping ‘Yes! Get in!’ of course. I feel personally guilty as being part of a generation of engineers that have allowed vehicles to get ever-fatter and heavier – taxing (or road-pricing) the extra flab might reverse the sloppy work of ‘my’ generation of engineers.”

These are the words of David Twohig, Cork-born car engineer, and a driving force behind cars such as the Nissan Qashqai, Renault Zoe, and the flyweight Alpine A110 sports car. As a man who instinctively hates waste, the idea of a weight tax is instantly appealing to him.

"There are precedents – you will know the Japanese kei-car mass, size and engine capacity restrictions. Japan also had, for many years, a vehicle width restriction of 1,690mm, above which cars were hammered with very high road taxes" Twohig told The Irish Times.

"You can even make a sensible argument that the heavier the vehicle, the more it damages and wears the road surface, so the more it should be taxed. But then my sensible boring engineer brain kicks in and I remember that unfortunately you can imagine a very efficient, very well-designed, but heavy vehicle. The Lucid Air is a great example – class leading efficiency in kWh/km, with a very clever and efficient powertrain and battery, but it weighs over two tonnes. It seems unfair to hammer such a thoughtfully engineered vehicle, somehow."

So what’s the solution? “First: reverse the bloody stupid current European law that actually increases the CO2 limits with mass – the fatter the car is, the more it can emit,” says Twohig.

"This is plainly silly, and was the result of a very powerful – and successful – German car industry lobby that fought for this increasing sliding scale, against a French car industry that was the very opposite. Of course, neither side were too concerned in reality about the planet, it's simply that Germany is successful in building large, expensive luxury cars, while France lives on small, light, cheap hatchbacks" he says.

“I would propose a combined tax: keep taxing CO2, as it helps to push for more efficient powertrains, aerodynamics and other clever tech. But either flatten the daft ‘CO2 vs mass’ curve, or, better again, reverse it, so cars are penalised for being too heavy. A sort of hybrid tax, measuring grammes of CO2/km and kilogrammes of weight. Of course, you could get clever and imagine a complicated mathematical taxing equation that would also hit NOx and particulates from things like brakes and tyres, but it could get so complex that only engineering geeks would understand it.”