Diesel vs Petrol: How Do They Compare?

The battle of diesel vs petrol has been raging for years, with each having its place and neither being able to land a conclusive blow. When you’re looking to buy a new car (no matter how many you’ve owned before), you must start by deciding which type of engine you need this time.

What should you pick if you need an SUV for heavy use, or if you only need a tiny car for short journeys? What about a company car that needs to be maximally efficient? In this post, we’ll set out the comparison as it stands today in the UK, weighing up the pros and cons of each fuel type (covering performance, pricing, and other factors) and helping you to make a choice.

The difference between petrol and diesel

When considering the fuels, remember that diesel and petrol are both refined from crude oil using fractional distillation. They simply differ in their exact composition: diesel is denser, more viscous, and less reactive under normal circumstances. This difference reflects the engine designs they’re formulated to suit.

Petrol is more explosive because it needs to suit the classic internal combustion engine: it gets mixed with air, compressed, and ignited with sparks. Diesel can be denser because it’s used in an engine that uses compression ignition: instead of using sparks, it compresses air so densely that it heats up enough to spontaneously combust.

The main difference, then, is in the type of engine being used. Each design approach has its advantages and disadvantages, and we’ll get to those in the next section.

Should you buy diesel or petrol?

Buying a vehicle is a big move with long-term consequences, so you naturally want to make the best possible decision. To that end, let’s see how these two engine types stack up:

Diesel: pros and cons

The biggest pro of diesel engines is that they’re more economical than petrol engines. The underlying mechanism is more efficient, and the energy density of the fuel means that less is required to cover the same ground (the compression ratio also means that they can offer more torque at low RPM). The CO2 production is also lower (something very important these days).

As for the cons, the cost is most notable. Not only is the fuel more costly (due to its energy density) but the engine variety itself is also more expensive. Where the CO2 is lower, the generation of NOx (oxides of nitrogen) is generally much higher, and those can lead to breathing conditions. The noise and overall experience are other concerns: some find diesel vehicles less pleasant to drive, but that will always depend on the specific engines involved.

Petrol: pros and cons

The main reason why people buy petrol vehicles is that they’re cheaper (as is the fuel required to run them) and more common, with those two things being closely related. They also provide driving experiences that are relatively smooth and quiet, even if that may largely be due to the design being significantly more mature. Give them the edge for power, as well.

You may want to avoid petrol engines because they consume fuel at a faster rate, and because there’s more reason to be put off by CO2 than NOx (the latter can be filtered fairly well anyway). In general, petrol vehicles depreciate faster than comparable diesel vehicles, so you might want to opt for the latter if you’re planning on selling down the line.

The UK Government is making petrol better for the environment, with lower carbon dioxide emissions, by switching standard petrol at all pumps to E10. Find out how this could affect your car and look up if it can use the new petrol – and what to do if it can’t.

Calculating petrol or diesel costs

There are two things you need to factor into this cost calculation. Firstly, there are the costs that go into buying and maintaining your vehicle, because cheap fuel doesn’t necessarily make something cheap to run. Secondly, there’s the actual price of the fuel required.

While the former will depend on the vehicle you end up choosing (and how you use it), the latter is reasonably simple to calculate. If you want to know what you can expect to pay for a particular trip, try using this journey cost calculator by filling out the current rates from whichever station (or stations) you’d use.

Hybrid, electric, and the future

Though we’re concentrating here on the comparison between petrol and diesel engines, those aren’t the only options. Engines that run partially or exclusively on battery power are becoming much more common, pushed along by the advancement of technology and the importance of environmental protection. With charging points going up across the UK, and the sale of combustion engines set to be banned in the coming decades, it’s a reasonable time to invest in an electric car (or a hybrid with a petrol or diesel backup engine).

That said, electric vehicles aren’t for everyone at this time, and whether it’s due to personal preference, convenience, cost, or specific needs that electric engines wouldn’t suit (heavy haulage, for example), you may want or need to stick with a petrol or diesel vehicle. If so, consider what we’ve looked at here when narrowing down your options.