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Earlier this month, we published a blog that looked at how you can save fuel and money whilst driving. Today, we take a look at exactly what fuel economy is and the most fuel-efficient cars that you can lease from Nationwide Vehicle Contracts.

It is an obvious statement that when it comes to saving fuel, electric cars are going to save you money at the fuel pumps, but these so-called green cars aren't for everyone. So what should you be looking for when it comes to fuel economy on a car?

Fuel Economy

Basically, fuel economy is described on Wikipedia as "the fuel efficiency relationship between the distance travelled and the amount of fuel consumed by a vehicle. Consumption can be expressed in terms of volume of fuel to travel a distance, or the distance travelled per unit volume of fuel consumed."

In respect of the first of those 'consumption expressions', we are looking at what is a car's MPG.

MPG is an acronym for "miles per gallon" and states how far a car can travel on one gallon of fuel. So, for example, if a car was capable of 50MPG, if you put one gallon of petrol in it, it would travel 50 miles before coming to a halt with a splutter. Be aware that this would not actually happen as MPG calculations are created in a laboratory, as you will see below, and are a guide rather than an actuality.

MPG is also, in Europe, a very British term, as it uses the old imperial measurements of miles and gallons. Across Europe you will find that fuel economy is expressed by “litres per 100 kilometres” (l/100km) instead. By using the mathematical data of 1mpg = 282.5l/100km, you should be able to convert if figures that you read are in either. In the USA, a US gallon is bigger than an imperial gallon (1.2 US gallons = 1 imperial gallon), so figures are different there too.

But you will need to know that, unlike may figures to do with your car, the higher the MPG, the more fuel efficient it is.

The alternative to MPG is an expression of fuel economy as a unit of fuel over a fixed distance, and usually as litres per kilometres. This is used in many European countries, China, South Africa, Australia and New Zealand.

How is fuel economy calculated?

In Europe, we have the New European Driving Cycle (NEDC) Test in place, which was introduced to provide consumers with a fair comparison of all cars available, and is in place to make sure that all fuel consumption figures are measured the same way, no matter the vehicle or manufacturer.

From this test can be calculated:

  • Urban fuel economy
  • Extra-urban fuel economy
  • A combined figure of the two
  • Carbon Dioxide emissions (CO2 Emissions)

When it comes to looking at a car, it is the two economy figures that people look at primarily, but in a world that is getting greener, the CO2 figures are also important - which is why car brands tell you all four figures when it comes to car specifications and official documentation.

So, just how is the MPG test carried out?

Initially, a car manufacturer will do tests on a computer to simulate vehicle journeys, before taking it out on the track and road to perform more tests. And then it goes to the NEDC people to do their tests.

It is important that all tests carried out are starting from the same base to ensure the fairest comparison possible, and this why all car tests are carried out at the same air temperature of between 20 and 30 degrees Celsius. When the tests take place, all cars are started from cold.

The Urban Cycle test (known as ECE-15) is there to simulate driving in a town or a city, and involves accelerating and decelerating very slowly up to speeds of nine, 15, and 20mph respectively - and then this is all done again three more times. This all takes place over a period of 780seconds, during which time the vehicle will have driven just over 2.8 miles .

It then moves on to the Extra-Urban Cycle which is tested by gently accelerating to 43mph, keeping the car at that speed for 50 seconds, then decelerating to 30mph, maintaining that speed for 69 seconds, and then accelerating back up to 43mph for another 50 seconds. After that, the car accelerates up to 60mph for 35 seconds, 75mph for 10 seconds, and then cruising slowly down to a complete stop. 

To get the Combined Cycle figure so often quoted in car specifications, the urban and extra urban official fuel consumption is added together and divided by two to give an average figure, and then the amount of emissions from the exhaust pipe are measure in grams per kilometre to give the CO2 emissions figure; this is available on a colour-coded "Green Rating" sticker has been available in the UK. Rated A means that A is less than or equal to 100 g/km, B: 100–120, C: 121–150, D: 151–165, E: 166–185, F: 186–225, and G: 226+. The various gases that are pumped out of a car's exhaust pipe include carbon monoxide, nitrogen oxides and hydrocarbons, although the one that is most worrying to the ecology is carbon dioxide (CO2).

For all cars involved, these NEDC tests are considered to be a good representation and an equal test across the board, although cars such as hybrids do benefit from the fact that gentle acceleration can mean that petrol doesn't need to be even touched at that time as the batteries regenerate thanks to the slow acceleration. This means that cars such as the BMW i8and Mitsubishi Outlander PHEV manage to get figures of 118 and 148mpg respectively - incredible figures that in reality are pretty unachievable in the real world.

MPG Figures

Here in the UK, the Advertising Standards Agency (the ASA) are always up in arms about fuel consumption and MPG figures being misleading, as they are not the same as driving 'in the real world', and also suggest that car manufacturers can use 'cheats' to 'massage' the figures. (This is a lot less commonplace now following the VW Emissions Scandal.)

Car experts across the world are also suspicious of the 'official' MPG figures that manufacturers release, and there are many websites on the internet (if you search for them) that use figures from real users to create a more real-world figure. In fact, a study by the International Council on Clean Transportation (ICCT), reported that the gap between official and real-world fuel-economy figures in Europe was 38% - an increase from the 2001 figure of 10% - with the difference between on-road and official CO2 values at 31% (an increase from 2001's 8%.)

Economical Cars

Here an example of some of the most economical cars that are NOT electric or hybrid that are available to lease from Nationwide Vehicle Contracts, although you can peruse a full list of 'Green' cars here that DOES include both electric and hybrid vehicles.

And as a general car model, these models are generally fuel efficient, with certain models bettering others in their grades.