Petrol vs diesel cars compared

Difference between petrol and diesel cars

When buying a new car, the traditional 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 – usually 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 more powerful and refined, while diesels boast greater efficiency and have more torque. However, significant developments in both types of engine have taken place over the past 30 years or so, 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.

Traditionally petrol cars are cheaper to buy but diesel cars have lower running costs. However, the margins are much finer now than in the past, and the difference in cost can vary from model to model to the extent that these 'rules' may be switched around in some circumstances.

The advances made in petrol engine technology in recent years - such as gasoline direct injection (GDI) and increases in compression ratios - mean that the gap between them and diesel 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. While they might not quite be able to match the lower CO2 emissions afforded by diesel engines, other tailpipe emissions are lower, making them a better choice for use in built-up areas.

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 pop to the shops in, or to use as a family workhorse? Do you normally drive in town or in the countryside? Once you've worked out your car's normal habitat, you can more 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 Titanium costs just £680 more for a 1.5 litre TDCi diesel, compared to the similarly powerful and efficient 1.0 litre EcoBoost petrol. The Volvo V40 Momentum on the other hand, commands a £1,860 premium for the D2 diesel over the T2 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 – Maserati's Ghibli saloon is a good example with the 3.0 litre petrol charged at £4,700 more than the 3.0 litre diesel – despite the latter costing less to run. Make use of NGC's Car Comparison Tool to help you find out the different costs between various models.

NGC's Car Comparison Tool

Which is cheaper to run?

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, with the main aspect being fuel economy. 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 likely to be your biggest ongoing cost.

Cars with higher MPG figures are cheaper to run, and these are usually diesels. This is where it gets tricky though, when margins get finer, generalisations get less useful, 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 from a fuel cost perspective. 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. New petrol engines challenge 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.

MPG - Fuel Economy

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. 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. The difference between a 45 MPG car and a 46 MPG car is worth around £21 per year, based on 8,000 miles annually with a fuel price of 120 p/litre.

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 NGC Fuel Cost Calculator via the button below.

NGC's Fuel Cost Calculator

Following on from the car's fuel economy in terms of financial importance is its emissions information. The reason for this is that VED - Vehicle Exise Duty, often known as car tax - is calculated in part using a car's CO2 emission figure. Currently there are 13 categories ranging 0 g/km CO2 to 255+ g/km, and these can greatly influence the car's First Year Rate.

First Year rates range from £160 for low-emission vehicles to £2,245 for those with CO2 emissions greater than 255 g/km. Thereafter, VED for petrol and diesel cars is simpler, charged at a flat £155 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 for petrol and diesel models increases to £490 each year for the Premium Rate for years two to six. Visit Next Green Car's car tax pages via the button below to search for cars by VED bands and for more information on costs.

NGC's Car Tax Microsite

Which is better for the environment?

This is a far from simple question. There are two sets of emissions that are most talked about in terms of engines - CO2 and NOx (nitrogen oxide) - both covered by European emissions regulations. While CO2 determines VED and BIK rates, NOx is soon to become a key consideration in paying for access to some urban areas many of which are likely to be designated as Clean Air Zones and surcharges such as London's Toxicity Charge or 'T-Charge'.

As a 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.

NGC's Company Car Tax Microsite

CO2 isn’t the only pollutant that needs to be considered, however, as there are a number of other gases that come out of an exhaust pipe. NOx emissions are gaining increasing amounts of coverage, particularly in the wake of the VW Diesel Scandal. NOx has a significant impact on air quality and the associated respiratory health problems that stem from or are exacerbated by air pollution. Diesel engines produce a higher amount of NOx than petrol engines as well as soot-like particle emissions.

Engine manufacturers are attempting to tackle these problems, though at a slower rate than first anticipated. Despite that, some of newest engines available are hitting their NOx Euro standard targest under real-world test conditions, as calculated by Emissions Analytics. Next Green Car uses Emissions Analytics data as part of its calculations for the NGC Rating where information is available, and the company's EQUA Index is able to show car buyers how models perform compared to official restrictions.

Emissions Analytics EQUA Index

Images of real world vehicle testing using PEMS and EQUA Index graphic courtesy of Emissions Analytics

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.

DPFs act like an air filter, trapping particulate matter to either then be disposed of when replaced, or to burn off collected matter via 'filter regeneration'. The latter either uses a catalyst or actively burns fuel to heat the soot to combustion temperatures. Filter regeneration takes place at higher speeds, where the vehicle is less likely to be in urban areas, and effective use can reduce soot emissions by 30-95%. Diesel cars remaining largely in urban areas without reaching higher speeds can see the regeneration process fail though, leading to clogged up filters with its efficacy significantly reduced.

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.

To find out the environmental impact of a specific model, use NGC's Emissions Calculator at the button below.

Which is best for the future?

There is no hard and fast rule as to which fuel type will prove the best long term bet to own. With the lower running costs normally associated with diesel engines, it might look the best option financially, but it is likely to be so only in the short term. Much will depend on how you use the car, and what penalties, surcharges, and restrictions will be put in place in the next few years as authorities tackle air pollution issues with increased vigour - such as the implementation of Clean Air Zones in the UK.

Thinking longer-term, only those who cover a very high number of miles each year, and almost never intend to drive the car into towns and cities should really still be considering diesel cars. Sales of diesel cars are already falling, and it is the above uncertainty - alongside the increased availablility and capability of alternatively fuelled cars - that is driving that decline. Not even mentioning the environmental issues, diesel-powered cars are going to be increasingly expensive to run in the near future.

It's still worth avoiding cars with unfashionable engines for their class if you want your car to hold its value well when it comes to selling it on - like an under-powered, small petrol engine in an SUV or a diesel unit in a sports car. But with so much uncertainty facing diesel drivers going forward, a better bet is likely to be petrol in a direct choice between the two fuel types.

However, to make your car-buying decision as future-proof as possible, you shouldn’t just consider petrol vs diesel but look at conventional hybrids, plug-in hybrids and electric vehicles. With government incentives, improving range, expanding charging networks and more choice than ever, an electrified powertrain will help reduce emissions and improve MPG. They are not going to be subject to anything like the same level of restrictions as diesel and petrol cars in the future, and they are significantly cleaner for the environment - should you pick the powertrain that suits your driving needs.

Head to Next Green Car's Electric Vehicles microsite below - or click on one of the links above for the sections on each technology - to find out more about EVs.

NGC EV microsite

Olly Goodall

Author:Olly Goodall
Date Updated:23rd Mar 2021

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