David Motton

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You've seen the stories, so next time you're in the market for a tug, what tow car will you buy? Diesel? Petrol? Hybrid? Electric? Let's look at the facts

Unless you've been living underground without a sniff of mobile reception and not even basic dial-up internet, you'll know that diesel is losing popularity.

Demonised in much of the mainstream media, and losing sales to other fuels, is it time for diesel tow car drivers to switch to petrol or hybrid power?

Or does diesel still make sense for the majority of tow car drivers?

What's wrong with diesel?

The big environmental concern with diesel cars is that they produce more local pollutants than petrol, and far more than a hybrid or electric vehicle.

It's true that the NOx emissions, which contribute to poor local air quality and can be harmful to human health, are higher with diesel-powered cars.

Under the current Euro 6 standard, a diesel car is permitted to produce no more than 0.08g/km of NOx. Petrol has a limit of 0.06g/km.

So, a diesel can emit a third more NOx than the equivalent petrol and still be approved for sale.

Looking at it in context

However, it's worth setting the current regulations in the context of what has gone before.

Under the Euro 5 standard, which applied until September 2015, diesel cars were permitted to emit 0.18g/km of NOx, so when Euro 6 came into force the level of emissions was more than halved at a stroke.

Particulates are another pollutant that tends to be associated with diesels rather than petrols. However, under the Euro 6 standard there is no difference in the permitted level of particulate emissions between diesel and petrol engines.

So, in terms of certain pollutants, a modern diesel does emit more than a modern petrol. But compared with older diesels engineered to earlier standards, today's diesels are much, much cleaner.

The 'dieselgate' effect

Of course, after the VW emissions scandal there's understandable doubt as to whether diesels are as clean as they promise to be in real-world driving.

Real-world independent testing by companies like Emissions Analytics shows that these concerns aren't unfounded, with many diesels – and petrols – exceeding official limits in real-world conditions.

However, these same tests also show that it is possible to engineer a modern diesel to meet emissions standards on the road as well as in the lab.

Ironically given Volkswagen's wrongdoing, Emissions Analytics tests have shown the latest VW Group engines to be among the cleanest on the road.

So, poor local air quality shouldn't be blamed on modern diesels. It makes more sense to point the finger at older vehicles and patterns of car use which see us rely on the car for even short journeys.

Will the new emissions tests make a difference?

New tougher tests should make it harder for manufacturers to build cars which fall well short of expected standards on the road.

Since the beginning of this month, any new car model being homologated (approved) for sale in Europe must be tested under the Worldwide harmonised Light Vehicle Test Procedure (WLTP).

It's still a lab test, but it's longer, involves harder accelerations, and greater top speeds than the old method.

Phase two

Just as importantly, it's now backed by a second test protocol, called the Real Driving Emissions test (RDE). As the name implies, this takes place on the road, effectively sense-checking the results of the WLTP.

There will be a margin of error allowed, so the performance in the lab and on the road won't need to be identical. For now, the RDE emissions could be 2.1 times the WLTP results, but that wriggle room will be reduced 1.5 times in September 2019.

That seems like a big margin, but keep in mind that Emissions Analytics results from last year showed some cars exceeding permitted pollution levels by 12 times.

So in reality, the new standards will force manufacturers to work harder to cut emissions and narrow the gap between lab results and those on the road.

Will I be hit with extra taxes if I buy another diesel?

The way cars are taxed has only just been subject to a big shake-up. Since April of this year, the amount of Vehicle Excise Duty is based on carbon dioxide (CO2) emissions for the first year only, before switching to a flat rate.

This first-year rate continues to favour diesel, as lower carbon dioxide emissions are usually an advantage of choosing a diesel over a petrol.

However, certain diesels will soon be charged for entering some city centres. For example, London's Ultra Low Emissions Zone (ULEZ) will come into force in the centre of the capital from April 2019 if Mayor Sadiq Khan's plans are confirmed (and certainly no later than September 2020). However, drivers of diesels meeting the Euro 6 standard will not have to pay.

There are plans for Clean Air Zones in a number of other towns and cities across the UK, but in most cases the expectation is these will follow London's lead and allow Euro 6 diesels to enter without a penalty.

This isn't to say that owners of the latest diesels will get away without any further charges or taxes. There's talk of increasing fuel duty on diesel in the Autumn Budget, although nothing had been confirmed at the time of writing.

Some councils are introducing additional parking permit charges for diesels, and in the case of the Islington council in London, the charge applies to the latest diesels as well as older models.

What about resale values?

There are signs that the resale value of diesel cars have taken a hit, but certainly not a wholesale collapse.

Over recent years, as a rule, diesels have held their value better than petrols. Now that's not always the case.

Take the Ford Mondeo 2.0 TDCi Titanium. Right now, What Car?'s experts predict it will retain 33% of its original price after three years and 36,000 miles on the road.

The equivalent petrol, the 1.5 EcoBoost Titanium, will be worth 34%, so 1% more than the diesel. A year ago, there was a 2% advantage in favour of diesel.

Of course that's just one model, but it reflects a genuine shift and real concern among car buyers about what's in store for diesel in the long term.

But, whether you look at the new or used market, demand for diesels has fallen rather than collapsed.

SMMT figures show that even after all the negative publicity around diesel cars, they still account for four out of every 10 new cars sold in the UK.

Is diesel still best for towing?

This is a crucial question, and one which makes the car buying decision of a caravanner very different from one made by the general motoring public.

Diesels make better tow cars. Better than petrols, better than hybrids, better than electric vehicles.

The day will come – and it may come soon – when a hybrid or an electric vehicle wins the overall prize at the Tow Car Awards, but in the event's 11-year history the winner has always been a diesel.

Why do diesels make the best tow cars?

The most important advantage a diesel has for a caravanner is torque, or pulling power.

It's torque that helps a car and caravan hold speed into a headwind. It's torque that means you can tow uphill without changing down several gears and wringing the engine's neck.

A diesel model's kerbweight is usually heavier than a petrol's because the diesel engine weighs more, and legal towing limits are generally higher for a diesel than the equivalent petrol.

Also, fuel economy when towing with a diesel is usually much better – if anything, the difference tends to be greater than in solo driving.

The petrol-diesel gap is narrowing

There's been a trend to fit petrol cars with relatively small capacity turbocharged engines – Ford's EcoBoost and Vauxhall's ecoFLEX engines are good examples.

These engines narrow the gap between diesel and petrol in terms of torque and fuel economy, but a gap does remain. For low-mileage tow car drivers or those who spend a lot of time driving in towns and cities they make viable alternatives to diesel.

But for regular towing, especially if you own a large and heavy caravan, diesel still works better.

More and more hybrids are approved for towing, and the best are capable tow cars. But our current favourite hybrid for towing, the Volvo XC90 T8 Twin Engine, is over £60,000.

The Mitsubishi Outlander PHEV is a more affordable hybrid for caravanners, but its towing limit of 1500kg is modest for a car with a kerbweight of around 1.9 tonnes.

As for pure electric vehicles, the Tesla Model X is approved for towing, with a legal capacity of 2270kg. But it costs upwards of £75,000. It's not a viable alternative to diesel power for the vast majority of tow car drivers.

Diesel: not dead yet

It would be wrong to ignore the problems inherent with diesel power. Even modern diesels have a greater impact on air quality than the equivalent petrol, even if the difference is not as great as it is sometimes made out to be.

And there's no doubt that diesel owners are going to be hit with more charges over coming years, although it's likely most of these charges will be levied on owners of older cars.

For drivers living in towns and cities whose journeys are mostly short commutes, petrol is generally a smarter choice than diesel – and with each new generation, hybrids and pure electric vehicles are improving at a startling pace.

But don't forget, the government's ban on the sale of new diesel (and petrol) cars is still well over 20 years away. It's too early to write diesel's obituary.

It may not be the right choice for all caravanners, but I predict that for most regular tow car drivers it will still be the preferred fuel for several years to come.

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