Diesel v petrol price: fuel for thought

Petrol engines use spark ignition to generate power. They draw a mixture of air and petrol into the cylinder, compress it, then ignite it with a spark plug. This creates a controlled explosion that pushes the piston down and turns the crankshaft.

Petrol engines tend to run smoother and quieter, delivering more horsepower and revving higher than diesel. They're the common choice for hatchbacks, smaller cars and performance vehicles where responsiveness matters.

Diesel engines work differently. They compress air to extremely high pressure, which heats it up. Diesel fuel is then injected directly into this hot, compressed air and ignites without needing a spark plug. This is called compression ignition.

Diesel engines produce more torque at lower revs, making them ideal for towing and motorway cruising. They're more economical too. Diesel has a higher energy density than petrol, so diesel cars can go further on each tank and typically deliver better fuel economy, especially on long motorway journeys where you'll see the best miles per gallon (mpg). That's why bigger vehicles like SUVs, pickup trucks and estate cars often run on diesel as standard.

When you're weighing up fuel costs, diesel tends to be pricier at the pump. There are a few factors behind this higher price, from how it's made to market demand. Here's what you need to know.

Diesel production involves a more complex refining process than petrol, which pushes up costs before it even reaches the forecourt. The oil used to produce diesel requires additional processing steps to meet strict quality and environmental standards, and this extra work means a higher price per litre.

Global oil market factors also play a part. Diesel is in high demand for commercial vehicles, lorries and heavy machinery, so when supply tightens, prices can climb further.

That said, diesel engines are more fuel-efficient, so despite the higher pump price, you could still save money over time if you're doing higher mileage. The main reason diesel costs more upfront is down to refining and demand, but those savings on fuel economy can balance things out in the long run.

If you own a diesel car registered before this date, you're more likely to be charged when entering clean air zones in city centres such as ULEZ in London, Birmingham or Bristol. These zones are designed to reduce pollution, and older diesel vehicles often don't meet the required standards.

So, if you're looking to buy a second-hand diesel car and you live or drive to places with clean air zones, this should be a cost consideration.

In the UK, there are typically two different octane-rated unleaded petrol options available: E5 and E10. The E stands for ethanol, and the number shows how much ethanol is mixed in the fuel as a percentage (5% and 10%).

E10 arrived in UK petrol stations on 1 September 2021. Most modern cars are designed to run on E10 fuel, which is usually the more cost-effective option at the pump. The ethanol is typically made from renewable sources such as sugar beet or corn, which makes the fuel more environmentally friendly.

If you're driving an older vehicle, it's worth checking whether it's compatible with E10. Some older models may need E5 or super unleaded instead.

Premium diesel is slightly different but follows a very similar idea. It contains a chemical called cetane, a substance which helps to increase the compression point of the diesel. Generally, the higher the rating of the diesel, the more expensive it is per litre.

For most diesel cars on the road, standard fuel is more than suitable.

Different brands of premium fuel will claim to have different benefits for your car. Some premium fuels claim to lower your fuel consumption, while others say they can help clean out your engine. Most will claim to increase performance, but this is heavily debated, and the only noticeable difference will be in performance or sports cars.

So, overall it's up to you whether you choose a petrol or diesel car. Both have their benefits and drawbacks, but it's always worth considering cost, environment and where you drive. Remember though, there's a plan to ban the sale of petrol and diesel cars in the UK by 2030, so it's worth considering this in your decision making.