One minute, the UK government is promoting diesel as the fuel of choice, and the next, diesel is apparently the worst thing in the world. What happened?

Well, the government promoted diesel for more than ten years, as diesel cars have better fuel economy, and lower CO2 emissions than their petrol equivalents. There was a big focus on cutting CO2 emissions, and diesel cars were seen as a way to do this. Consequently, diesel car buyers were rewarded with lower road and company car tax.

However, in the past couple of years, several things have happened that have turned the world against diesel. First, there was the Volkswagen #dieselgate scandal, when emissions tests were rigged by the company. Cars’ emissions and fuel economy figures were much worse in everyday use.

The government also needed to start tackling the high levels of pollution in cities. It is repeatedly breaking air pollution limits and getting sued by the European Commission.

Diesel engines produce nitrogen oxides (NOx), which contribute to pollution. There are more nitrogen oxides produced by older diesel cars and other vehicles, so the government announced there would be higher charges for people driving the most polluting diesel engines in urban areas at peak times.

Then, in November 2017, Philip Hammond’s confusing Budget statement suggested the cleanest new diesels would be hit hardest with a sales tax in the future. Which, understandably, put even more people off buying them.

If you do a large number of miles each year (15,000+), and most of your driving is on motorways or dual carriageways, it still makes sense to buy a diesel car over a petrol or hybrid, because of the impressive fuel economy you’ll get.

As diesel has gone through a bit of a crisis of late, there are some really good discounts available on diesel cars too. So much so, that it will likely offset any further taxes that might be planned in the coming years.

Diesel cars do tend to cost more than petrol equivalents, cost a bit more to service, and the fuel costs more. But if you are doing a lot of miles, you’ll hit the ‘break-even’ point, where the costs cancel each other out. It is important to do the maths though, and work out if the additional cost will be worth it for you.

On top of that, petrol and diesel cars do feel different to drive. Diesel cars have a strong pull from low down in the rev range, so they’re more relaxing to drive. Because of the strong pull, diesel also helps if you regularly carry lots of passengers or cargo, or tow a caravan.

If your driving is mostly around towns and cities, and you do a lot of low-speed driving, a diesel is not recommended. Apart from the tailpipe emissions, it’s likely you’ll clog your diesel particulate filter (DPF), which filters out soot and NOx. Although there are ways to unclog it, such as a good blast on the motorway, it might not work, and you could end up with the car at the mechanics, and a hefty bill.

It’s true that diesel cars emit more particulates than petrol ones, but in general, CO2 emissions are lower by around 20%.

However, older diesel cars have much higher particulate and CO2 emissions than newer cars. The levels allowed have been increasingly tightened by the European Union since 1992.

New cars sold currently have to meet Euro 6 emissions standards, which means 99% of all soot particles are removed from the exhaust. The cars have to be fitted with DPFs, most newer diesels have AdBlue tanks (find out more about AdBlue), and have various systems which convert NOx into nitrogen and water.

New rates of car tax were introduced in April 2017. Find out more about car tax bands, and how much your car is to tax.

For the first year of tax, the amount is calculated differently to consequent years, and it’s based on the car’s CO2 emissions. From 1 April 2018, diesel cars which do not meet new emissions standards have moved up one car tax band.

The government wasn’t raising as much money in road tax as it could under the previous system, as cars that emitted less than 99g/km CO2 used to qualify for free road tax, but don't any more.

Most people will still be making the choice between petrol or diesel, but a hybrid or electric car could be a great choice. Find out more about the pros and cons of buying a alternative fuel car, and the pros and cons of petrol and diesel.

From 8 April 2019, the T-Charge has been replaced by an Ultra Low Emission Zone (ULEZ). Any diesel car that doesn’t meet Euro 6 emissions standards (and petrol cars that don’t meet Euro 4 standards) will have to pay an extra £12.50 a day to drive through certain parts of London. That means diesel cars registered before September 2015, and petrol cars registered before 2006 will be hit by the charges. Find out more about ULEZ.

ULEZ is in addition to the Congestion Charge, which is an £11.50 daily charge for driving a vehicle within the charging zone between 07:00 and 18:00, Monday to Friday. Find out whether or not you need to pay the Congestion Charge.

It's also likely other cities around the UK will introduce similar ULEZ schemes in the coming years.

Searches for petrol vehicles are close to overtaking diesel searches. In June 2018, diesel searches fell to 47%, continuing a long-term decline. This is compared to June 2016 when diesel accounted for 71% of searches.

Alternatively fuelled vehicle searches currently stand at 4% – a figure which has only gone up 2 percentage points in the past two years.

The fall in diesel searches is due to confusion and a lack of clarity on the situation. But also, as well as moving away from diesel, consumers are still sticking with a fuel type they know and understand – petrol.

This is possibly why average CO2 emissions from cars are set to rise for the first time in two decades, according to figures from the Society of Motor Manufacturers and Traders (SMMT). Diesel engines emit on average 20% less CO2 than their petrol counterparts.