Conventional petrol and diesel have less impact on the environment if they are highly fuel-efficient – in other words, if they have high fuel economy, 'miles-per-gallon' or 'mpg'.
Most motorists assume that conventional petrol and diesel engines with small cylinder capacity are more fuel-efficient, with better 'mpg'. However, for each engine size fuel economy can vary widely, and is dependent on how well the engine is designed, built and 'tuned'.
Although a car's fuel efficiency depends on many factors, vehicle weight plays a particularly important role – the lighter weight, the better the fuel economy. In general, the effect of vehicle weight on fuel-efficiency is more important than engine size.
In recent years, fuel economy in European vehicles has gradually improved, in part due to European CO2 legislation for all new cars sold in the EU, a law designed to reduce carbon dioxide emissions from new cars. CO2 reduction has also been driven by changes in car buyer preferences following the fuel price peaks in 2008 and the on-going economic downturn – both of which have renewed interest in smaller 'city' cars and superminis with better 'mpg'.
Using data from the Society of Motor Manufacturers and Traders (SMMT), the sales weighted average CO2 of new cars (as measured on the official test) in the UK is approximately approx. 128 g/km – which equates to an official combined fuel economy of around 48 mpg for petrol cars and 54 mpg for diesels.
These averages hide the large variations in fuel economy across vehicle classes that range from 91 mpg for the most fuel-efficient diesel (currently the Peugeot 308), to 18 mpg for one of the most polluting SUVs.
However, it is worth noting that you need not necessarily trade off vehicle size and/or safety to get a car with good fuel economy. The SMMT has calculated that "if the lowest CO2 emitting vehicles in each segment were used then average CO2 emissions would fall by 30 per cent".
Therefore, whatever type of car you are looking to buy or use, it is worth checking the fuel economy label which is displayed alongside all new cars in the showroom, and using the new car search to find detailed information about the car's fuel economy (mpg), emissions and fuel costs. Alternatively use the following if you'd prefer us to do the legwork for you:
Quick search: Top 10 best MPG petrol and diesel cars
Engine technology is also an important consideration regarding fuel economy. In order to comply with future emissions standards, new petrol engine technologies have been developed and are beginning to appear in production cars. One of the most important is Gasoline Direct Injection (GDI) (also known as Fuel Stratified Injection FSI) which reduces fuel use and vehicle CO2 emissions by up to 20%. Hybrid electric cars also offer significant improvements in fuel economy (of around 25%), and in a real sense, are considered as the next technological evolution of 'conventional' cars.
Higher 'miles-per-gallon' implies less fuel use, which in turn means lower fuel costs. For an average petrol or diesel car, and assuming petrol and diesel pump prices at around £1.40 per litre, for each 5 mpg improvement in fuel economy, the fuel savings are around 2 pence per mile (worth £200 per year for the average motorist).
The important fact to remember here is that you need not trade off vehicle size, performance, comfort and safety to get a car with good fuel economy. SMMT's findings (see above) suggest that fuel cost can be cut by at least 30% without having to choose a smaller car, but instead by switching to a more fuel-efficient model.
Download SMMT pdf: SMMT CO2 Report 2014
Currently, all petrol and diesel vehicles have to pay the daily London Congestion Charge.
For the reasons discussed above, fuel-efficient petrol and diesel models can be found in all vehicle classes, and are available from most vehicle manufactures.
All that is required is for you to check the fuel economy label, displayed in the showroom alongside every new car, which shows the official combined fuel economy and CO2 emissions for each model. The label is also now being provided at an increasing number of used car dealerships.
For an up-to-date comparison of the most fuel-efficient petrol or diesel cars available in the UK please use the new car search.
Spark-ignition (petrol) engines utilise the four-stroke cycle. During the induction stroke a small amount of fuel and air are drawn into the cylinder. The petrol-air mixture is then compressed into a small volume and then ignited by an electrical spark from the spark plug. The explosion causes the gases to expand (power stroke) forcing the piston and turning the crankshaft. The burned gases are then expelled from the cylinder (exhaust stroke) via the exhaust valve.
Whereas older designs used a carburettor to mix the fuel and air before combustion, newer engines employ electronic fuel injectors to provide the correct amount of petrol. In order to comply with EU legislation, three-way catalysts are fitted new petrol car exhausts – these use precious metals to catalytically reduce the amount of carbon monoxide, nitrogen oxides and unburned hydrocarbons from the exhaust.
Although diesel engines are also four-stroke, only air is compressed in the cylinder instead of an air-fuel mixture, and at the end of the compression stroke the fuel is directly injected into the combustion chamber. Once the diesel is injected into the cylinder it immediately vaporises and spontaneously ignites, due to the high air temperature caused by compression. This combustion process produces a mixture of hot gases that then drive the piston.
The latest diesel technologies include common rail injection and advanced diesel turbo-chargers. In common rail systems, the injection pressure is independent from engine speed and load. This enables the injection parameters to be freely controlled leading to reductions in engine noise and NOx emissions. Diesel after-treatment systems have also been developed to comply with new Euro standards – these include diesel particulate filters (DPFs); now fitted to the vast majority of new diesel cars.
Focusing on greenhouse gas emissions (such as CO2), higher 'mpg' implies less fuel use, which in turn means fewer emissions of carbon and less environmental impact. For example, a small diesel city car capable of 86 mpg (3.3 litres/100km) emits only 88 gCO2/km, whereas a petrol 'gas-guzzler' with 18 mpg capability (16.0 litres/100km) will emit 376 gCO2/km.
Considering regulated emissions, the link between fuel economy and vehicle emissions is far weaker – in other words, some small cars with good fuel economy may emit as many particulates or NOx as larger cars – a general rule is hard to identify.
Engine type also has an impact on the level and type of vehicle emissions. The fuel efficiency of diesel engines is higher than for petrol units due to their higher combustion temperature. Diesel also has slightly higher energy content than petrol per unit volume. As a result of the differences in fuel composition and engine conditions, petrol and diesel cars differ in their relative emissions performance – petrol vehicles emit fewer NOx and particulates, and diesel vehicles produce 15%-20% fewer CO2 emissions. To some extent, there is a trade-off between reductions in local and global emissions.
The unique Next Green Car Rating used on this site provide an accurate comparison between specific models and fuel types. Indeed, one of the reasons that Next Green Car developed the NGC Rating system was precisely to address the complex scientific issues outlined above.