This might surprise a few people, but diesel cars still have an important role to play in the UK's transition to emission-free transport, because they're so very efficient.
While diesel sales have plummeted in recent years, there are still several brilliant models out there that are ultra-effective at sipping fuel.
When it comes to reducing our reliance on oil and shifting to a more sustainable future, diesel cars remain a key component.
Why are diesel cars better now?
Two things have helped diesel cars clean up their act: particulate filters (known as DPFs) and diesel exhaust fluid.
A DPF collects the soot that’s produced by burning diesel and stops the vast majority of it being emitted from the tailpipe.
This collected soot is periodically burnt off at very high temperatures.
Diesel exhaust fluid – also known as AdBlue – is used to reduce a diesel car’s nitrogen oxide (NOx) emissions.
It’s automatically injected into the exhaust system after the DPF to convert much of the NOx to nitrogen and water.
Diesel cars with both of the above emission-reducing tech have a Euro 6 emissions rating, and all new diesel cars sold from September 2015 have had to be Euro 6-compliant.
The engine below illustrates how many pollution-fighting bits are included these days.
Why were diesel cars so popular?
The UK government decided to change the taxation system in the early 2000s to a carbon dioxide-based ranking.
This is because when burned, diesel emits almost half of the CO2 compared with the same amount of petrol – great news for our fight against climate change.
Back then, however, without emissions-reducing tech, diesel vehicles were emitting a lot more particulates and NOx than petrol engines, which were bad for human health.
While CO2 has a global impact, NOx and particulates affect a vehicle’s local area – both are breathed in and cause all manner of long-term health issues.
It wasn’t long before the EU brought in regulations to tighten up diesel emissions, and we’re now at Euro 6, introduced in September 2014 (a year before all new diesel cars had to be compliant).
The latest version of this is Euro 6d, which applied to all new cars from January 2021.
The car industry’s trade body in the UK, the Society of Motor Manufacturers and Traders (SMMT), says: "It would take 50 new cars today to produce the same amount of pollutant emissions as one vehicle built in the 1970s" – which just goes to show how far we’ve come.
What are the most economical diesel cars?
A car’s fuel economy will vary depending on who’s behind the steering wheel, because we all have different driving styles.
However, the following diesel cars are listed by their official average fuel economy rating, measured using the worldwide harmonised light-duty vehicles test procedure (WLTP).
Mercedes-Benz E300 de – 217.3mpg
No, we haven’t made a mistake – the E-Class plug-in hybrid diesel really does have a frankly astonishing official economy figure.
You’ll need to keep the car’s battery topped up to achieve it. Do that, and you’ll be able to run this used Mercedes on electric power for up to 33 miles, according to the brand itself.
Peugeot 208 1.5 Blue HDi – 71.4mpg
Launched in 2019, the Peugeot 208 is one of the sharper-looking superminis out there, with five doors, fancy LED lights and a 1.5-litre diesel engine that’ll return more than 70mpg, officially.
The baby Peugeot’s interior is equally interesting, with a very stylish dashboard including instruments that you view over the steering wheel, rather than through it.
Vauxhall Corsa 1.5 Turbo D – 70.6mpg
It’s no coincidence that the 2019 Corsa shares its good looks with the Peugeot – both brands fall under the Stellantis umbrella.
The Vauxhall Corsa also shares the Peugeot’s engine and other oily bits, which explains why its economy figure is so similar.
The main difference between this used Vauxhall and the Peugeot above is its styling, which is subjective – choose whichever you think looks the best.
Skoda Octavia 2.0 TDI SE – 68.9mpg
Just about any version of the Octavia is a cracking family car, thanks to masses of interior space (including a huge boot, especially on the Estate model), but this version also adds terrific economy to its repertoire.
This used Skoda estate doesn't have the most powerful engine but it has a lot of torque, which means you’ll be able to leave it in its top gear on the motorway and cruise without having to change down to overtake.
Volkswagen Golf 2.0 TDI – 68.9mpg
A bit like the Peugeot and Vauxhall, above, the Skoda and VW share many parts.
This version of the Mk8 Golf has the same 150hp diesel engine and six-speed manual gearbox.
Plus, you can find used Volkswagen models at impressive prices.