In order to make them run cleanly and efficiently, diesel engines need more technology than petrol. As a result, diesel cars tend to be pricier to buy than an equivalent petrol model, whether new or second-hand. The cheapest diesel version of Britain’s best-selling car, the Ford Fiesta, costs £15,395 new. The cheapest petrol Fiesta is £13,795. That’s a £1630 difference. Assuming the Fiesta driver was covering 6000 miles per year, it would take nine years to make that up in fuel savings. Even if they do 12,000 miles annually, it would still take four and a half years.
For the vast majority of motors registered before April 1, 2017, car tax or Vehicle Excise Duty is worked out according to carbon dioxide (CO2) emissions. This gives diesel a slight advantage. Take the example of the Ford Fiesta. With a 1.25-litre petrol engine, it costs £30 a year in road tax. Plump for a similarly priced 1.5-litre diesel and you’ll pay no tax at all. But that is about to change. From April 1, 2017, the way new cars are taxed will change. In the first year, they will pay a rate governed by CO2. After that, it will be a flat £140 annual fee.
Winner: Diesel (at the moment)
Diesel cars can frequently cover more miles between services than petrol. However, if you don’t do big mileages, a diesel car could become like a needy child. The reason is, most modern diesel have a component called a Diesel Particulate Filter. This sits in the exhaust and catches harmful sooty particles. However, they need to get to a certain temperature to burn off the soot or they get blocked. On short journeys or bumbling around town at low speeds, the exhaust doesn’t get hot enough and the filter gets clogged up.
For every driver, fuel is one of the biggest costs of motoring. Here diesel is at a significant advantage as it’s a lot more efficient. Using our Ford Fiesta as an example again, the 1.25 petrol model returns 54.3mpg; the 1.5 diesel a whopping 78.5mpg. If unleaded is £1.20 per litre, a driver who covers 6000 miles a year will pay £603 in petrol. The diesel owner will only pay £424 at £1.22 a litre. Obviously, the more miles you cover, the more savings you’ll reap. Our diesel Fiesta driver would save £358 a year in fuel compared to their petrol driving friend over 12,000 miles.
Although diesel has low emissions of CO2, the gas blamed for climate change, it hides a dirty secret: NOx. These Nitrogen Oxide particles have been deemed responsible for respiratory diseases. To improve air quality, some councils around the country are declaring clean air zones. Drivers of older diesel cars will have to pay a tariff for driving into city centres. That said, the majority of modern diesels are clean enough to not be affected by this.
Although the government spent years urging us to buy diesels, the tide has turned. Diesels are now being demonised. There’s talk of a scrappage scheme to encourage people to get rid of dirty diesels and buy alternative fuel vehicles such as hybrids. There are also fears that the used value of diesels will collapse. But talking to industry experts, this seems like scaremongering. Nonetheless, petrol seems to have a much rosier – and less controversial – future.
Although petrol wins this poll four-two, it’s a narrow victory. The future for diesel is uncertain but industry experts agree that demand should remain healthy for used diesel car market, making drivers’ investments sound. And the extra mpg that diesel drivers reap does still ensure the fuel from the black pump makes more sense over petrol the more miles you do, especially when comparing used cars that cost the same.