Petrol vs diesel cars compared

Difference between petrol and diesel cars

When buying a new car, the choice between petrol and diesel power can be complicated. Most buyers will be looking for an efficient car, with high fuel economy and low emissions – traditionally measured in the UK as miles per gallon (MPG) and grams per kilometre of carbon dioxide (g/km CO2).

There used to be huge differences between petrol and diesel engines. Historically, petrol engines are faster, have greater levels of horsepower and are more refined, while diesels boast greater efficiency and have more torque. However, significant developments in both types of engine have taken place over the past 25 years, since the European engine emission standards came into place in late 1991. This has seen engine manufacturers focus not only on improving the natural strengths of petrol and diesel units, but also working on their flaws. Now we see petrol engines with higher levels of torque than ever before while some diesels have become so refined that it can be difficult to tell what fuel they use.

Which is better – petrol or diesel?

To decide which is best for you in terms of petrol vs diesel, there are a number of different factors to consider, including fuel economy, and how much the car costs to buy.

Normally petrol cars are cheaper to buy but diesel cars have lower running costs. This is particularly true at the moment as the price of diesel is not just close to that of petrol - as it has been for some time. That said, the advances made in petrol engines recently mean that gap between them in terms of efficiency is very close. Ford’s Ecoboost and Fiat’s TwinAir petrol units for example are just about as efficient as the equivalent diesels.

To ensure you make the right decision, you also have to work out how you use, or intend to use, your car. Do you cover short distances or long? Do you use a car to just pop to the shops in, or to use as a family workhorse? Do you normally drive in town or out in the country? Once you've worked out your car's normal habitat, you can accurately make the best decision between petrol and diesel.

Which is cheaper to buy?

Broadly speaking, diesel models are more expensive to buy than equivalent petrol versions. This can vary anywhere between a couple of hundred pounds to well over £1,000. For example, Ford's Focus Style costs just £350 more for a 1.5 litre TDCi diesel, compared to the similarly powerful and efficient 1.0 litre EcoBoost petrol. The Mercedes Benz A180 SE on the other hand, commands a £1,425 premium for the diesel over the petrol.

Buyers don't always pay more for diesel power though, and it tends to be towards the top end of the market where the roles are reversed. Petrol power is still preferred for sporty cars and this means the diesel engine often proves to be the cheaper alternative – Audi's A5 coupe is a good example with the 3.0 litre petrol charged at £950 more than the 3.0 litre diesel – despite the latter costing less to run. Visit Next Green Car's Car Comparison Tool to help you find out the different costs between various models.

Which is cheaper to run?

So diesel cars tend to be more expensive to buy, but this is off-set by the fact that they usually cost less to run. After the initial outlay, one of the next biggest factors when deciding which car to buy is running costs, and there are two main factors to this – fuel economy and emissions. The more miles per gallon your car will do, the less time and money will be spent on filling the tank back up again, and this is your biggest ongoing cost.

Following on from the car's fuel economy in terms of importance is its emissions information. The reason for this is because VED is worked out using a car's emission figures – g/km CO2. Currently there are 13 categories ranging 0 g/km CO2 to 256+ g/km, and these can greatly influence the car's First Year Rate.

Prices for the first year range from £0 for zero-tailpipe emission vehicles, to £2,000 for those with CO2 emissons greater than 255 g/km. Thereafter, VED for petrol and diesel cars is simpler, charged at a flat £140 for the Standard Rate each year, unless the car - regardless of powertrain - has a list price of more than £40,000. If this is the case, VED increases to £450 each year for the Premium Rate, for years two to six. Visit Next Green Car's car tax pages to search for cars by VED bands and for more information on costs.

Those cars with higher MPG are cheaper to run, and these are usually diesels. This is where it gets tricky though, when margins get finer and you are really looking for the best car for you. On the whole diesels offer better economy figures than petrols, but to determine whether it is best to pick a diesel car over a petrol depends on what you are going to use it for.

If you're going to cover a lot of miles – particularly on motorways and A roads – a diesel car is almost certainly going to be your best bet. Likewise a small petrol engine will be best in the city with lots of short journeys. This is because of how the engines work. A petrol engine revs more freely and is able to get within its efficient powerband easily. This is better when constantly accelerating away from traffic lights or out of junctions, especially in a small and light car. Petrol engines have come in and challenged the stronghold diesel had on the market and this is borne out by the fact that most of Next Green Car's Top 10 Best MPG city-cars are petrol powered.

On the open road however, the lower revolutions per minute of a diesel engine, combined with the fact that torque is available lower down the rev-range, makes for a more frugal and relaxing drive. With the diesel engine spinning slower, the unit uses less fuel and emits fewer g/km of CO2.

One constant variable is fuel price which not only fluctuates around the country but also comparatively between the two fuel types. It is more common for petrol to be cheaper per litre than diesel but this isn't always the case and the simplest way to make sure you are spending the least amount possible on your car is to pick one that will return the highest number of miles per gallon. A word of warning though, quoted fuel economy figures are almost never achievable so it would be worth looking at a car’s calculated Real MPG and using the Next Green Car Fuel Cost Calculator.

Which is better for the environment?

This is a far from an simple question. The emissions that are most talked about in terms of engines are CO2 levels. This is because CO2 has long been the biggest focus for European emissions regulations, and it is the figure that sets how much VED and company car tax (BIK) you pay.

As a good general rule, diesel engines produce fewer CO2 emissions than petrol engines. The smaller the engine, the closer the CO2 performance though, so you may find that one manufacturer’s petrol powered city car emits less CO2 than a rival’s diesel model. Go up a few model sizes and you would be very hard pushed to find comparably powered petrol and diesel engines where the latter doesn’t produce lower CO2 emissions. It is because of this factor that most company car drivers pick a diesel powered car. Visit the Next Green Car Company Car Tax microsite to work out the best company car for you.

CO2 isn’t the only pollutant that needs to be considered though as there are a number of other gases that come out of an exhaust pipe. NOx (nitrogen oxide) emissions are gaining increasing amounts of coverage as it has one of the most noticeable effects – it is one of the key gases in the build-up of smog. Diesel engines produce a higher amount of NOx than petrol engines (though this can be reversed as the engines get older) and diesel units also produce soot-like particle emissions.

Engine manufacturers are tackling these problems, though at a slower rate than first anticipated. Despite that, there will be phrases such as NOx traps, exhaust gas recirculation (EGR), diesel particulate filters (DPF) and selective catalytic reduction (SCR) whizzing around when you are looking a buying a new car.

NOX traps use a NOx absorber to reduce exhaust gas emissions, though the material – which acts as a molecular sponge – is still being developed to make it efficient. The main problem is that, just like a sponge, once it has absorbed as much NOx as it can it is then useless until the NOx trap is replaced.

SCR is an exhaust after-treatment technology that uses a catalyst to break down NOx. This comes in the shape of an additive called AdBlue, but this needs to be kept topped up for SCR to remain effective. EGR uses exhaust gases as part of the air mix that is pushed into the engine’s cylinders. With lower oxygen levels, the NOx produced is reduced. However, the technology only works effectively when the engine is under low-load levels.

Which is best for the future?

There is no hard and fast rule as to which will prove the best long term bet to own. With the lower running costs normally associated with diesel engines, it would probably be the best option – but it does greatly depend on how you use the car.

In terms of depreciation, diesels will hold their value better against a similarly used petrol model, but they tend to be bought to tackle higher mileages so this would bring their price down. The biggest consideration is to avoid cars with unfashionable engines for their class if you want your car to hold its value well, like an under-powered, small petrol engine in an SUV or a diesel unit in a sports car.

To try and make your car as future-proof as possible, perhaps you shouldn’t just consider petrol vs diesel but look at conventional hybrids, plug-in hybrids and electric vehicles too. With government incentives, improving range, expanding charging networks and more choice than ever, an electrified powertrain will help reduce emissions and improve MPG.

Chris Lilly

Author:Chris Lilly
Date Updated:11th Sep 2015

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