Petrol or diesel? When the time comes to buy a new car, this is one of your biggest decisions. In making your choice, you’ll need to consider several factors, which go beyond the price of fuel at the pump.

Fuel costs

Filling up your car costs more if you opt for a diesel motor.

According to the most recent data from the AA, in May, the typical cost of petrol rose by 3.4p to 128.8p per litre. However, this is still substantially cheaper than a litre of diesel, which rose by 1.3p over the month, to hit 134.3p per litre.

When filling up a 40 litre fuel tank, you’ll pay £51.52 for a petrol vehicle and £53.72 for diesel. While that might not seem like a big difference at first, it adds up over the lifetime of a car.

Fuel economy

Your choice between petrol and diesel will affect just how much bang for your buck you get when it comes to fuel economy.

The consumer champions at Which? have been monitoring the rates of fuel economy dating back to 2012. They have found that, on average, diesel motors offer drivers around 52.8 miles per gallon, compared with around 44.9 miles per gallon with petrol motors.

Concerns over emissions

Diesel cars have come under intense public and media scrutiny over concerns about their emissions. This was thrown into the limelight in 2015 during the Volkswagen ‘dieselgate’ emissions scandal.  

Diesel cars, especially older ones, usually produce more nitrogen oxides (NOx) than petrol cars. NOx is a significant contributor to pollution and can cause deadly respiratory diseases.

While there is no imminent UK ban planned for diesel cars, under the government’s 2017 UK Air Quality plan, the sale of both new diesel and petrol cars will be stopped in 2040.

There are also other implications of the backlash against diesel emission.

Emission charges

One is London’s new Ultra Low Emissions Zone (ULEZ), which mostly applies to diesels that don’t meet certain European regulations. They, and some older petrol models, will have to pay an extra £12.50 a day ‒ on top of the congestion charge ‒ for venturing into the city centre. The area covered by the ULEZ will expand in October 2021 to include a much bigger London area, right up to the North Circular Road at the top and down to the South Circular.

Other cities across the UK are also taking action to improve air quality and reduce pollution.

Tax

Concerns over diesel emissions have also had an impact on car tax.

All drivers pay car tax, based on their vehicle’s emission levels. However, if you have a diesel car that doesn’t meet the latest diesel emission standard – introduced across Europe for all cars last year to protect people from exhaust gas pollution – then your tax rate moves up by one band.

For example, say you have a car which emits 51-75 g/km of carbon dioxide. If this is a petrol vehicle, your first-year rate will be £25. But if you have a diesel which doesn’t meet the new standards, that first year tax rate will be £105, even if your emission level is the same. After the first year, you’ll pay the same rate of £140 a year, whether your car is petrol or diesel.

Cars registered before 1st April 2017 are subject to the old car tax scheme, which doesn’t make a distinction between petrol and diesel, with the rate you pay entirely depending on your emissions levels.

Reliability

When car shopping, it’s important to research the reliability of your chosen vehicle.

While this will vary significantly between individual models, research published last year suggests that diesel cars may be less reliable, and then cost more to put right.

Car maintenance firm MotorEasy crunched the data on 30,000 faults on three to eight-year-old petrol and diesel cars over a 12 month period, and found that not only were diesel models three times more likely to go wrong, they also came with an average repair bill of £517, compared to £433 for petrol models.

Purchase price

According to the RAC, diesel models are likely to cost you anything from £1,000 to £2,500 more than the petrol equivalent, partly due to the extra tech needed for them to pass emissions tests.

Demand

According to figures from the Society of Motor Manufacturers and Traders (SMMT), there have been 289,273 registrations of diesel motors in the year to date, down by more than 18% on the same period last year, with registrations having now fallen for 26 consecutive months.

In contrast, sales of petrol cars have increased by 3.6% from 669,142 to 693,318.

This drop in diesel car sales is an important factor to consider when looking for your next car. While the lower demand may mean you find it easier to negotiate a bargain on a diesel car, you may then find it harder to sell it down the line.

Go to Solved to read more about driving, the rules of the road and road safety.