Petrol Vs Diesel: What Should You Go For?

A white and red petrol pump at gas station
Image source: Marco Verch on Flickr

As soon as you start learning to drive, you realise that the world of cars is full of choices. PDI or ADI, manual vs automatic, intensive course or hourly lessons—there are a lot of decisions you need to make! It's important to know that this doesn't necessarily end when you earn your licence. In fact, you'll be faced with even more options when it comes to purchasing your first car.

Starting with the absolute basics, you need to decide how you want your car to be powered. In other words, do you want to drive a petrol or diesel car? Unless you're a motor enthusiast or physics whizz, you might not know what kind of things you need to consider when making this choice. Lucky for you, PassMeFast is here to help!

We're about to delve into all of the important differences between petrol and diesel cars. Our guide leaves no stone unturned and, by the end of it, you'll be fully equipped with the knowledge needed to make the best decision for you. So, let's get on with it!

The fundamental differences

The terms petrol and diesel apply to the type of engine within a car and the fuel that the engine requires. Both are made and work in different ways.

Fuel

Petrol pump

Let's start with the science! What we know as fuel starts out as crude oil that is heated in a furnace. The heating process causes fractional distillation to occur, which means temperature levels vary across the length of the furnace. The bottom of the furnace is the hottest point, and the top the coolest. 

Petrol is produced in the top section of the furnace. This results in it having a lighter carbon chain and lower boiling point. Diesel, on the other hand, is produced in the middle section. As a result, it is denser in consistency and has a higher boiling point.

Still with us? Good, because there's only one more sciency bit to get through...

Engine

Close up open car bonnet
Image source: Chad Kirchoff on Flickr

In terms of the actual mechanics, diesel and petrol engines both rely on the internal combustion method. They differ, however, when it comes to how this combustion occurs. A petrol engine produces power as a result of air and petrol being compressed by a piston and then ignited by a spark. Diesel fuel, however, is not flammable enough for this process to result in combustion. Instead, a diesel engine compresses air until it is ready to combust, at which point the diesel is added and ignition occurs.

Because it takes more effort to maintain power, diesel engines are larger and heavier than petrol engines. This is why you'll find them in bigger cars and lorries—they need a sturdy structure to support the intense vibrations that the combustion process causes.

Money matters

Yellow cartoon car with pile of coins

Fuel efficiency

One of the biggest reasons people opt to buy a diesel car is that they are more fuel efficient than petrol cars. Diesel itself is denser (packing more energy per unit) and diesel engines use a steady ignition process based on high levels of compression, which results in more heat being produced—thus more mechanical energy is created. In fact, it is estimated that diesel cars use 15-20% less fuel than petrol cars.

Don't run off to buy that diesel car just yet, though, as some of the potential savings are cancelled out at the pump. Yep, while diesel fuel is more efficient than petrol, it's also more expensive. Typical!

Car prices & maintenance

When it comes to recently qualified drivers, petrol cars are a more popular choice. A big part of this is probably the fact that petrol cars tend to be cheaper to purchase than diesel cars.

Not only that—they're also cheaper to maintain. You see, because diesel engines work that little bit harder than petrol engines, they're more susceptible to wear and tear. Those who invest in a diesel may find, therefore, that they're having to fork out more frequently for repair work.

Emissions & taxes

Exhaust fumes from a muddy red car

A major consideration any car buyer needs to make is what exactly their vehicle is emitting. Even if you don't care about your impact on the environment (time to get with the programme, mate!), others do enough that it can affect your bank balance.

Diesel cars have gone on a bit of a journey when it comes to how they are viewed from an environmental standpoint. While diesel cars do produce a dense black smoke of smog and soot, they don't emit as much carbon dioxide (a major contributor to global warming) as petrol cars. This meant that, for a long time, diesel cars enjoyed a spot in a lower tax band than their petrol counterparts.

The problem is, while petrol cars produce more carbon dioxide and carbon monoxide, diesel cars produce nitrogen oxides, which are linked to issues like acid rain and potentially fatal breathing disorders. As we become more educated on such issues, the tax differences between the two types of car have begun to narrow and, in some cases, even reverse.

In April 2017, the rules were changed so that owners of both diesel and petrol cars paid the same rate of tax from the second year of ownership (the first year was still based on CO₂ emissions). As of April 2018, however, any diesel cars that don't meet the standard known as Euro 6 will be charged a higher rate of tax in the first year. A lot of cars made before September 2015 were not designed with this standard in mind.

Extra penalties

cartoon silhouette of police officer

As diesel cars develop a bad reputation amongst the eco-conscious, extra charges are cropping up in major cities too. In 2015, for example, the London Borough of Islington introduced a £2 surcharge on all diesel cars. Ouch!

Similar schemes like the Ultra Low Emissions Zone in central London also tend to hit diesel drivers hard, though they stick to the Euro 6 standard rather than applying a blanket charge.

Diesel particulate filters

Another result of the environmental concerns surrounding diesel cars has been the introduction of Diesel Particulate Filters (DPF). They are designed to reduce the amount of soot emitted by diesel cars and all new models come fitted with them.

While this is of course a positive step, it does add an extra worry if your diesel car is due an MOT test. Not only must your car be fitted with a DPF, it must be in good working order and have no evidence of any tampering on it. This is not a huge deal, but it does constitute one more thing that could cause you to fail an MOT.

Remember those frequent repairs we were talking about? Well, diesel particulate filters have a tendency to get clogged if your car is not used frequently at high speeds (i.e., on the motorway). If it gets so bad that it needs replacing, you could be looking at a bill going well into the thousands of pounds, according to Which?.

What's your driving style?

Aerial view of motorway surrounded by trees

Petrol cars have smaller engines and are able to accelerate and decelerate quickly, making them particularly suited to the stop and start nature of city driving. You'll also find that petrol cars respond much more sharply to your control. So, if you mainly use your car for shorter journeys and like to feel a lot of power over the vehicle, petrol is probably the way to go.

Diesel cars, in contrast, are highly suited to long car journeys where you'll be maintaining a fairly steady pace. This is a great way to make the most (read:  reap the financial benefits) of the fuel efficiency of diesel cars. Plus, if you spend quite a bit of time on the motorway, you'll be burning off any excess soot that might be sitting in your DPF. Win-win!

With that big engine also comes a lot of pulling power. Diesels produce more torque, which means they are able to carry heavy loads without the need for high revs. This makes them an ideal car to use to tow trailers or transport bulky cargo.

Diesel: Pros and cons

  • More fuel efficient, making them cheaper to run
  • Large, powerful engine produces more energy with lower revs
  • Able to pull heavy loads
  • More expensive to buy and fuel
  • Tend to require more repairs over time, which can also result in higher insurance premiums
  • Produce dangerous emissions like nitrous oxides
  • May face higher taxes and extra charges in cities
  • The larger engines are louder and so contribute to noise pollution (though this isn't much of an issue with newer diesel vehicles)
  • Uncertainty about the future of diesel cars is bad for reselling prospects

Petrol: Pros and cons

  • Cheaper to buy and fuel
  • Engine not as prone to wear and tear issues
  • Don't produce some of the dangerous emissions associated with diesel cars
  • Less likely to face extra charges when driving in cities
  • Not as fuel efficient as diesel cars
  • Produce significant levels of CO₂ and CO which are major contributors to climate change
  • Don't produce as much torque as diesel engines

Diesel vs petrol: which should you go for?

Cartoon red car with woman driving attached to car key

Whether you decide to go for a diesel or petrol car is up to you! Just ensure you're making an informed decision. Who knows, maybe the final verdict will be: neither! Alternative energy vehicles like electric cars are becoming increasingly accessible and popular as we turn our attention to protecting the planet. As a result, petrol and diesel may soon be a thing of the past!

Whichever you choose, remember to hit the road with a safe and positive attitude. Happy driving!