With fuel prices high, more and more motorists are asking which type of engine can help keep fuel costs down.
With fuel prices always higher than we'd like, many drivers are weighing up their options when it comes to filling their tanks and asking: which is more economical – petrol or diesel?
Of course, when you choose what type of fuel your motor runs on, efficiency is only one factor. Car buyers also want to know how their vehicle will perform, how servicing costs are affected, and what the impact could be on resale values.
Many buyers believe that diesel cars represent better value. So are they right? How do petrol and diesel cars compare?
Purchase costs and resale value
Generally speaking, for two similar cars the diesel model is likely to be a bit more expensive, but it is thought that the “diesel premium” has come down in recent years.
If you need to sell your vehicle on, however, diesel cars tend to hang on to their values better than their petrol rivals. The current rise in demand for diesel vehicles will help keep resale values high, as will diesel’s lower carbon emissions given the government’s lower car tax rates for green vehicles (see below).
Filling your tank and fuel efficiency
At the end of 2015, the average litre of petrol in the UK cost 102p and the average diesel litre was 106p a litre.
So why is diesel considered a more economical option? Well, this is down to the fact that diesel cars tend to be more fuel efficient and do more miles per gallon.
However, the advantage varies from model to model, so you’ll need to use a calculator to see how much your fuel will cost you.
Other things being equal, the more you drive, the better value (in fuel terms) a diesel vehicle is likely to be – provided forecourt prices for diesel don’t become significantly more expensive than petrol.
But fuel is not the only aspect of a vehicle’s running costs: what about routine repairs and services?
In the past, diesel servicing has been viewed as more expensive but, according to Which? Car, the fact that diesels need servicing less frequently means that costs even out in the long run.
How you drive
Another issue that needs to be borne in mind is the type of driving you do: will your car be out on the motorway a large chunk of the time, or will it be used for journeys in busy towns and cities?
The AA says that diesel engines used to be better suited to short journeys because they were energy-efficient earlier in the journey.
But newer diesels fitted with diesel particulate filters (which help cut soot emissions) need to be driven at higher speeds from time to time to clear the filters.
If the filters are not cleared automatically by the car, it could mean an expensive trip to the garage.
The AA’s current advice is that diesels with diesel particulate filters are suitable if the bulk of your driving is on the motorway, whereas a small normal petrol engine is more appropriate for short local journeys.
If you’re buying a used diesel, check whether it has a diesel particulate filter.
Advances in diesel technology mean that the traditional view of petrol engines as being faster, smoother and less noisy no longer always holds true.
The best advice is to take the vehicles you’re interested in for test drives to see which you like most – if you can notice any difference, that is.
Another plus point for diesels is the fact their lower carbon emissions make them cheaper to tax.
Which? Car points out: “The 1.4-litre diesel VW Polo BlueMotion, is exempt from car tax charges under current rules. But the equivalent, 1.4-litre petrol VW Polo is hit with an annual bill of £125.”
Before you buy, use the government’s vehicle excise duty calculator to see the difference in tax bands for the models you’re considering.