It took a long time for diesel cars to become popular in the UK, sales increases coming about largely because of tax incentives for fleet drivers. Until recently the rise of diesel seemed unstoppable, then Volkswagen was caught being dishonest and suddenly petrol-engined cars are back in favour once more. If you’re confused as to which way to go, read on…
 
The driving experience
Diesel cars are now so refined that a clattery engine is no longer a valid reason for avoiding one. What makes them so appealing is the extra muscle they provide; a diesel engine is inherently more torquey than a petrol one and it’s torque that gives you that effortless acceleration.
As a result diesel cars are ideal for towing and they’re also perfect motorway cruisers because the engines don’t have to work as hard at high speeds. Indeed, it’s hard to think of a reason to avoid buying a diesel from a driving point of view, apart from the fact that they don’t rev like a petrol engine does – or sound as great at the redline.
However, diesels do take longer to warm up, so if you do mainly short journeys you might find that in the winter the engine never gets warm and as a result the heater never works. That might seem annoying but it’s worse than that because most fuel consumption and engine wear takes place when an engine is cold.

Green considerations
This one is quite complicated. Diesel engines emit less CO2 than the petrol alternatives because they’re more efficient. A litre of diesel contains more energy than a litre of petrol which is why petrol engines are inherently more thirsty – although new technologies are making them significantly more efficient.
What muddies the waters is the cocktail of exhaust emissions that go with burning diesel fuel – specifically particulates and nitrogen oxides (NOx). The latest diesel engines are far cleaner in this respect, but that’s because they’re now much more expensive to make and complicated, which has a knock-on effect on reliability and running costs.
 
The fiscal argument
It’s no good just looking at the fuel economy here – which is what too many people do. You have to look at all of the running costs which means insurance, road tax, initial purchase costs, depreciation plus maintenance too.
The initial hurdle is the purchase price because not only do diesels cost more to make and therefore to buy, but discounts are often more readily available on petrol-engined cars. It’s normal for a diesel car to carry a 10 per cent premium over a petrol equivalent – so about £1,000-£2,000 if it’s a small family hatch. That much money buys a lot of fuel…
However, while you’ll pay extra to buy a diesel car, you’ll get a proportion of that premium back when you come to sell, as the car will also be worth more than its petrol-engined equivalent. Insurance tends to be a bit more expensive for diesels but road tax costs are usually lower – but not by enough to be a deciding factor.
At the time of writing, the two fuels cost much the same to buy – but diesel has cost an extra 10p per litre before now. With diesels being more economical you won’t have to buy as much fuel of course but it’s worth doing the sums before buying – what sort of mpg can you expect from a petrol model, and how does that compare with the diesel alternative?
Maintenance costs don’t tend to vary that much between petrol and diesel cars – in theory at least. In reality tyres can wear faster because of the heavier engines (petrol engines are lighter) but a diesel engine doesn’t have an ignition system to worry about, so there are no spark plugs, HT leads or anything else to replace. But read on and you’ll see that there are plenty of other things to go wrong – things that can cost a lot more to fix…
 
What about reliability?
There’s more to go wrong with a diesel-engined car and it’s this added complexity that can have a big effect on running costs. To comply with emissions regulations a diesel particulate filter (DPF) has to be fitted and this will fail if the car isn’t given a good run regularly.
There are also turbochargers, dual-mass flywheels and EGR valves – some of which are fitted to petrol engines, but all diesel engines have these. While turbos tend to last pretty well nowadays, EGR valves and dual-mass flywheels are a weakness that can cost plenty to fix.
 
So what’s the verdict?
It may be that you have no choice. Some SUVs come with a diesel engine only because car makers have realised there’s no appetite for petrol. It’s the same for some family cars; did you know for example that the Volkswagen Passat, Citroen C5 and Peugeot 508 all come with diesel power only? On the flip-side, many small cars and sports models come only in petrol form, because the cost or characteristics of a diesel engine aren’t suitable.
If you do have a choice, how you use your car is likely to be the deciding factor. If you do lots of motorway miles, you’re towing or you’re buying a big, heavy car that needs plenty of muscle a diesel engine is almost certainly the way to go. But if you cover relatively few miles each year and most of those are in an urban environment you’re probably going to be better off with a petrol-engined car.
Still not sure? Then don’t focus on the financial side because the jury is still out on what is ultimately the cheapest to run. Likely reliability is also too unpredictable for any car and only you know how important the green argument is. Which leaves the driving experience. So try out both petrol and diesel models, work out which one you prefer, and just enjoy using it.
 
Richard Dredge
March 2016