Petrol or diesel - which fuel should you choose?

Petrol or diesel? It's a decision that the vast majority of UK car buyers make and one that some spend more time mulling over than others.

There are petrol fans who would never consider crossing over to the dark side and diesel lovers who quiver at the thought of reaching for a green-handled pump but the majority of motorists have no strong allegiance.

Most will just use the fuel that's best for them but identifying which one that is has never been trickier. Here, as part of our Green Car Guide, we're aiming to throw some light on the subject.

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Background

Historically, UK motorists only chose diesel if they covered lots of miles and didn't mind their cars sounding like poorly-maintained farm machinery. Everyone else went for petrol. There were other options but almost universally, they looked extremely silly in comparison.

Then the world seemed to wake up to the dual facts of a dwindling oil supply and the environmental damage we were causing by burning it. The push was on to cut our use of fossil fuels and control the output of harmful emissions from our vehicles.

Since environmental issues became huge news in the car industry, the pace of change has been rapid. Great strides have been made in alternative-fuel vehicles but, despite significant sales growth, they still only account for a tiny segment of the market. Petrol and diesel reign supreme and if anything, the efficiency improvements we've seen in conventional cars have been even more remarkable.

So both petrol and diesel cars are better than ever but that doesn't help customers struggling to choose between them. Indeed, diesel claimed a 50.6% share of the market in July 2010, sneaking past petrol in the UK for the first time, so the loyalties of UK car buyers appear to be split down the middle. The question is, where should yours lie?

Technology

There's nothing new about the basic internal combustion engines that drive our petrol and diesel cars. Manufacturers have been manufacturing them for as long as they've been churning out cars, so they should be getting pretty good at it by now.

The technology has been continually honed and developed over the entire lifetime of the motorcar but growing environmental concerns and spiraling fuel costs have upped the pace of change in recent years. Today's top engines are hugely advanced and achieve levels of performance and efficiency that seemed impossible a decade ago.

If you haven't already, it really is time you wiped the old diesel stereotypes from your memory banks. The advent of turbo diesel engines then, more recently, common-rail fuel injection technology went a long way towards making noisy, dirty, sluggish diesels a thing of the past.

The best modern diesel engines are competitive with equivalent petrols in most of the key areas. They also retain their traditional strengths on grounds of fuel economy and pulling power.

The prowess of the best diesel engines may have put petrol's long-held position as the favourite fuel of UK car buyers under threat but a fight-back is underway. The latest high-tech diesel units aren't cheap and many manufacturers are looking to small capacity petrol engines, often with turbocharging and direct fuel injection, as an affordable alternative in compact cars.

Environment

With their higher combustion temperatures and the greater energy content of diesel fuel itself, diesel engines are more fuel efficient than petrol units. By burning less fuel, diesels also release less of the carbon dioxide (CO2) contained in it. This gives them another key advantage over the petrol alternatives.

There's more to green motoring than thrifty fuel economy and low CO2 emissions though. Compared to petrol cars, diesel models emit higher levels of carbon monoxide (CO), nitrogen oxides (NOx) and particulate matter (PM).

These substances have negative impacts on air quality and the greenhouse effect but there's less of a link between the amount of fuel burned and the quantities of them released. Unlike with CO2, even relatively fuel-efficient diesel cars can have high levels of CO, NOx and PM in their exhaust gases.

In the UK, where fuel prices and CO2-based taxation systems are the main concerns of motorists, these other exhaust emissions might not seem like a big deal. In reality though, there's tough European legislation controlling their output. Diesel cars often need advanced combustion systems and exhaust after-treatment measures to comply with these standards and this plays a major role in pushing their prices higher.

Emissions legislation is a key reason why diesel cars are so popular in Europe and yet have been largely ignored by consumers in Japan and America. Were European standards focus on cutting the CO2 output of cars, these other markets have a different approach which is tougher on NOx and the other exhaust emissions that diesel models produce more of.

Driving

There's a huge amount of variation in the way different petrol-engined cars drive and the same is true of diesel models. There are, however, certain characteristics that we associate with vehicles that are powered by each fuel.

Improvements in diesel engine technology have closed the refinement gap so that these days, it's more of a trench than a chasm. The best modern diesel engines are only fractionally less refined than equivalent petrol units, with the old-school diesel clatter usually well supressed.

Diesels typically offer more torque as well, making the majority of their pulling power available lower in the rev range - right where you want it for stop-start urban driving and muscular in-gear acceleration.

Despite this, for enthusiastic drivers, the free-revving character, sweet soundtrack and sharp throttle response you get from a great petrol engine takes some beating. The vast majority of sports and performance cars still run on petrol although some memorable fast diesels have made it to market.

Costs

For most people, the decision on whether they should choose a diesel- or petrol-engined car boils down to a financial calculation. The key question is: are they going to cover enough mileage for the diesel model's fuel savings to cover its higher list price?

As a rough guide, car buyers can expect to pay between £1,000 and £2,000 more for a diesel engine than they would for a similar petrol unit. On a £30,000 car, that isn't a big proportion of the overall price by when we're talking about £12,000 superminis, it's a lot of extra money to stump up.

A diesel model will be cheaper to run but it might take years to recoup that price premium in fuel savings. It depends on the amount of mileage you do and changes in fuel prices can have an impact on your calculations.

Generally speaking, diesel is harder to justify for buyers of smaller cars who tend to cover less mileage and are more sensitive to price increases. This situation is likely to be perpetuated in future as emissions regulations grow stricter and the cost of making diesel cars that comply increases.

Diesel makes more sense for higher mileage drivers thanks to its fuel economy. Company car users and businesses are exposed to CO2-based taxation structures which don't affect private buyers, so the low CO2 emissions of diesels are particularly beneficial for them. Many diesel cars also have stronger residual values than their petrol counterparts because of their low running costs and that counts in their favour.

Petrol or diesel?

In the end, there's no easy answer to the petrol or diesel conundrum. Car buyers need to weigh up their own priorities and the kind of driving they're likely to do.

The low running costs and abundant torque of diesel needs to be balanced against the low up-front costs and free-revving smoothness of petrol. On-going improvements in engine technology and fluctuating fuel prices will also have an impact.

What we can say with reasonable certainty is that, barring miraculous developments in the field of alternative fuels, motorists will continue to face the old petrol or diesel problem for some time to come.

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