Diesel vs Petrol: Which is the Best?

Emily Hanson 10 minutes Published: 28/04/2021

Let me introduce you to Stephen and his wife Michelle. They both have jobs in the city centre. Their house is just a short walk from the train station and from there, it's just a quick 20-minute journey to work. They're lucky because Stephen's family live a quick walk away and all their amenities are close by. On occasion, they go on road trips to see their friends up in the Highlands but apart from that, they don't really get much use out of their car. What a nice life Stephen and Michelle have, eh?

Today is a sad day for Stephen. It's time to say goodbye to his ancient petrol Vauxhall Corsa, a relic from Stephen's late-teen boy racing days. Maggie (as he lovingly named it) has served Stephen well over the past almost-decade but Michelle has always hated it. She was so glad when it failed its MOT. When the mechanic gave them a quote for repairs it made both of their eyes water, so Stephen has finally accepted that perhaps it's not worth saving the Corsa anymore. Bye, Maggie.

They've had their eyes on a brand new Kia Ceed on lease, but they're a bit confused about fuel types. Do they want a petrol or diesel engine? What's the difference between the two of them? Neither of them have bought a new car for years and back in the day, 18-year-old Stephen didn't even stop to think about such minor details. If it was red, he wanted it.

The choice between petrol and diesel fuel shouldn't be a snap decision - there are loads of things to take into consideration to find the best vehicle for your needs. Fuel efficiency, your driving habits, car tax, performance, and the environment should all be on your list of things to consider. If you're like Stephen and Michelle and you've got no clue what sets the two fuel types apart, check out this guide and decide for yourself which is the better fuel for you.

What is petrol?

Are you ready for a mini science lesson? Well, you're getting one anyway.

Way back when, sea animals and plants died and their bodies were covered in tonnes of sand and silt. Over the course of hundreds of thousands of years, intense pressure and heat built up, transforming their remains into fossils. These fossils broke down into hydrocarbons and then formed oil-saturated rock. From this crude oil, petroleum, a liquid chemical, was extracted through a process called fractional distillation. Petrol comes from petroleum.

In a petrol car, spark plugs are used to ignite the fuel. When it's ignited, a piston is moved and then a series of machinery works together to propel the car forward. 

What is diesel?

Now that you're basically an expert in petrol extraction, you'll get the gist of how diesel is made too. Diesel is a type of distilled petroleum but in the process of fractional distillation, it's found lower in the spectrum. It's a dirtier and heavy fuel with a high boiling point. This boosts its fuel efficiency above that of petrol engines.

In a diesel car, air is compressed inside a cylinder until the pressure combusts the fuel itself without the need to be ignited manually. This form of combustion is more powerful than a petrol engine, but it's quite hard-going on the ol' engine so diesel engines are more likely to need regular maintenance.

Diesel vs Petrol: Fuel Efficiency

When you're comparing diesel and petrol cars, you'll want to take a close look at their fuel efficiency, which you'll see from their MPG (the miles the car can do per gallon of fuel). There is a noticeable difference between the fuel efficiency of petrol and diesel cars, with diesel cars coming out on top.

NimbleFins have done some of the hard work for you and calculated these average fuel efficiency figures:

  • Petrol: 36 MPG
  • Diesel: 43 MPG

It's worth bearing in mind that, when you're looking at two models, the MPG figures are almost certainly calculated under perfect, pristine conditions. Manufacturers want their cars to look amazing, so they carry out these fuel consumption tests in labs with no headwinds, traffic jams, or idiots who break suddenly without warning you. These "official figures" are to be taken with a pinch of salt, but the point still remains that diesel is more fuel efficient.

If you're keen to know how fuel efficient a car is in the real world, we recommend you check out the What Car? True MPG calculator or Honest John’s user-generated Real MPG section.

So, in terms of fuel efficiency, in the diesel vs petrol battle, diesel reigns supreme.

Diesel vs Petrol: Affordability

We’ve all shuddered as we’ve noticed fuel prices spike up yet again at the pump. If one type of car is cheaper to run than the other, surely you’d want to go for that one?

But it’s more than just fuel costs you need to account for. To work out the overall affordability of each fuel type, you need to consider: the cost of fuel, tax, depreciation, insurance and maintenance.

Fuel Prices

As of July 2019, petrol costs around 127.7p per litre, whilst diesel is slightly more expensive at 131.1p per litre. But - and this is a big but - if you turn your attention back to the fuel economy comparison, you'll see that diesel actually ends up cheaper in the long-term.

Let's say you're on the open road for about 7,000 miles per year. When Stephen and Michelle compare the cheapest petrol and cheapest diesel Kia Ceed models, they found that their annual fuel costs would be:

  • Petrol car (52.3mpg): £781.52
  • Diesel car (74.3mpg): £563.75

Here, it's clear to see that diesel gives you far more bang for your buck. The engine sips on the fuel much slower than a petrol car, so it'll last you a lot longer. 

However, if you're like Stephen and Michelle and you barely use your car, you're not going to reap the benefits of this because you'll be using less fuel. You might find that the lower cost of petrol suits you better for your infrequent trips to the petrol pump.

Tax Rates

Not so long ago, diesel motorists enjoyed lingering in the rock bottom tax bands for their cars. Before April 2017, all rates were based on official CO2 emissions, so if yours emitted less than 100g/km, it was exempt from tax. Diesel cars emit less CO2 than petrol, so tax was basically always cheaper.

Since 2018, the rules have changed and diesel cars are actually charged pricier tax rates since diesel cars emit much higher levels of other toxic gases, like nitrogen oxides. In a bid to reduce air pollution, unless your diesel car meets Euro 6 standards, expect to pay higher rates.

So, how does this work on a practical level? Say your car emits 110g/km of CO2. If it's a petrol, you'll pay £150 in your first year. If it's a diesel, it'll cost you £170. Doesn't seem too drastic a difference, huh? But go further up the emissions scale to 135g/km, your petrol car will cost you £210, but a diesel will sweep up a whopping £530! Diesel cars that don't meet RDE2 standards are heavily penalised when it comes to taxation, and the savings on your fuel costs can quickly be sucked up by your tax bill.

Depreciation

Depreciation is a dirty word for car owners. Can you feel the shivers down your spine? Within your first year, your car can have halved in value, so it's really not a great investment.

In general, diesel cars cost £1-2.5K more than their petrol equivalents. This higher price tag is stamped on them because manufacturers require a lot of extra tech to get them through their emissions testing. For Stephen and Michelle, the cheapest Kia Ceed with a petrol engine costs £18,600, whereas the cheapest diesel costs £19,850, a £1,250 difference.

Diesel cars cling more tightly to their original value after time. If resale value means a lot to you, then diesel cars are better in terms of their residual value.

Insurance and Maintenance

If you remember, earlier on we said that the combustion process is much rougher on diesel cars so they usually require more maintenance work. Since they're more likely to need work done to them, they're seen as more risky to insure, so insurance costs can be steeper for diesel. On average, diesel cars can cost 10-15% more to insure.

In terms of maintenance, there is a biggy to watch out for: the diesel particulate filter (DPF). The DPF captures exhaust soot to reduce emissions. The filter only has so much room to store the soot, so you need to burn it off by driving very fast for a period of time. This is called "regeneration". If you take regular, short journeys at low speeds, the DPF can become blocked, puffing out black smoke. This will have you fail your MOT and it's very expensive to fix - we're talking thousands!

You don't have this issue with a petrol car. The maintenance costs with diesel cars can pretty much wipe out any fuel savings you make. If you’re not one to hit the motorway on a regular basis, you’d be at risk of racking up some serious maintenance and repair bills with a diesel car.

Overall Costs

Let's break it all the expenses down and see what Stephen and Michelle would be best off doing.

Petrol Kia Ceed

  • New Purchase Price: £18,600
  • First Year Tax: £170
  • MPG For 7000 Miles: £777.01
  • Total For 1 Year: £19,547.01
  • Total For 5 Years: £23,235.05

Diesel Kia Ceed

  • New Purchase Price: £19,850
  • First Year Tax: £150
  • MPG for 7000 Miles: £561.50
  • Total for 1 Year: £20,561.50
  • Total for 5 Years: £23,387.50

After 5 years, the diesel vehicle is slightly more expensive than petrol. If Stephen and Michelle were just looking for a car to keep and trade in in a few years, the running costs of a petrol would suit them well. In the long-term, it may be a better financial decision to go with diesel. This is, however, not accounting for any potential maintenance issues and their insurance.

Diesel vs Petrol: Performance

"Yeah, yeah, yeah, I get the money part. Which is the better fuel for top speeds?"

A decade ago, the difference between petrol and diesel engines in terms of performance was clear as day. Older diesel cars had a much smaller power band, giving them a load more torque spread over a shorter rev band. Without getting into the nitty gritty of mechanics, this basically means that they're good for towing and shifting heavy cars. But the noise was absolutely awful.

Petrol vehicles, on the other hand, were much more nimble and offered a smoother ride, and they didn't clatter and clang like diesels.

The main difference between the two is that diesel cars have greater pulling power, perfect for long-hauls or motorway driving, whereas petrol cars are smoother and have sharper responses, better for city driving with stop and start traffic or on unpredictable rural roads.

Modern diesel cars have seen much improvement in recent years. Car manufacturers are closing the gap between the two fuel types by developing new diesel cars with petrol-like responses and petrol counterparts with torque to compete with diesel.

Petrol engines are better suited for manual driving, needing revved up to hit peak performance, but diesel cars are good for automatic cars because their enhanced torque isn't designed for revving in low gears.

Stephen and Michelle just need a sturdy, reliable car to cart them to the shops and back, and sometimes to up North. Petrol seems to suit them best for the performance they need.

Diesel vs Petrol: The Environment

Now one for the treehuggers: how do these fuels fare in terms of air pollution?

Whilst diesel cars emit lower CO2 levels than petrol cars, they produce over 11 times more Nitrogen Oxide and Nitrogen Dioxide fumes. NOx can inflame our airways, causing respiratory disorders. 

In the past, petrol had lead in it, but this too caused cancer and respiratory issues, so in 2000 it was banned. Petrol still has higher carbon dioxide emissions but it's cleaned up somewhat in the past two decades. Diesel is the one that the British Government are most heavily penalising for its harmful impact on the environment and our health.

Neither of these fuels can be sustainably burned without emitting toxic fumes, so they'll both be completely banned by 2040 in the UK. In their place, we'll see low emission hybrid plug-ins and zero emission electric vehicles. This petrol vs diesel competition will be put to bed when these electric cars roll around but that's still a good 20 years off. 

For now, it seems that petrol is the marginally cleaner fuel and it's definitely Stephen and Michelle's pick for their suburbanite community.

Diesel vs Petrol: The Final Verdict

It's been a tough fight and I'm afraid it's still quite a mixed answer to our petrol or diesel dilemma.

Short-term, a diesel car is going to cost you more. Diesel fuel is pricier, tax is higher, and insurance and maintenance costs are bigger. Only when you use your car for long-haul journeys will you benefit from its higher MPG, which will save you fuel money over time. Diesel cars also hold onto their original value more than petrol cars, so if you're looking for something with a semi-decent resale value, diesel is your fuel.

It also depends on what you're looking for in terms of performance. If you take to the motorway on a frequent basis, a diesel car is much better suited to you, but if you tend to take sporadic trips to the supermarket, you should opt for a petrol.

And if you're suffering from a bit of climate anxiety, petrol is the safer option of the two.

Stephen and Michelle are going for a petrol car. They don't drive enough to benefit from diesel's enhanced MPG and they don't need some powerful machine to take to the highway with. All they want and need is a simple vehicle to take them from A to B.

Now you've read all of this - what will your next car be?