The truth about fuel consumption

MT FLEET CAR SPECIAL: Official figures are based on unrealistic driving conditions and can be a poor guide to real mpg and running costs. Now there is a new high-tech approach that leaves no room for doubt about the best - and worst - performers for your fleet.

by Chas Hallett
Last Updated: 09 Oct 2013

Filling up the tank can be the biggest expense of running a vehicle, especially with fuel prices as high as they have been in recent months. Which means that whether you're paying for it or the company is, fuel consumption is the hottest of hot topics in the fleet business these days. And both drivers and fleet managers are increasingly noticing the gap between the mpg that is officially claimed for any given car and what it actually achieves on the road.

The fact is, however carefully you drive, you are unlikely ever to match the mpg that was quoted in the brochure, in the ad, or, more than likely, by the salesman. And we'll wager that horrible realisation came within days of taking delivery just through the number of times you have to fill up, or from what your car's (usually reasonably accurate) trip computer is telling you.

This is the number one frustration of any new car buyer. Which is why MT sister title What Car? has launched a scheme to give more accurate 'real-world' fuel consumption figures. Called True MPG, it uses state of the art equipment and testing on real roads to give a more accurate picture of what you can expect, rather than the manufacturer's figures, which derive from ideal laboratory conditions.

So why are there such big differences between real-world figures and the 'official' ones advertised? Well, just as schools have become more adept at coaching pupils to pass their A levels, so car-makers are getting better at engineering cars to do well in the fuel economy tests. In both cases, these improvements are not necessarily reflected by real-life experience.

All manufacturers are legally obliged to subject their cars to exactly the same test (see panel, opposite) and the results are all entirely above board. But some cars can be more readily electronically tuned to perform disproportionately well in the tests. And these cars are often the ones sold primarily as cheap runabouts.

The True MPG figures burst this bubble. A three-cylinder Kia Picanto is a good example of a car that has an impressive headline figure, but when you use it in mixed conditions, on real roads with real drivers, it doesn't deliver it.

The Kia has a claimed combined mpg of 67.3. But in real-world driving it returns a mere 41.2 mpg, the reason being that it does well on the rolling road in a laboratory. But drive it at varying speeds and for long periods on the motorway and it can never give you what you expected. So if you bought this car to save on fuel you'll probably end up feeling short-changed.

The same applies to the Toyota Prius, the original hybrid beloved of celebs such as Cameron Diaz and Tom Hanks. Like any petrol-electric car, it's supremely economical in town (probably why minicab companies love them too), but in real-world conditions a government figure of 70.6 mpg turns into a not-so-green 52.2 mpg. And even that drops to 45.6 mpg if you drive it reasonably spiritedly on the motorway.

However, not all the cars tested are quite so off the target - larger-engined specimens have tended to fare better in the True MPG tests, perhaps because fuel consumption is not such a hotly contested prize at this end of the market. One, the Mercedes S350 CDI limo, has even exceeded its 36.7 mpg official government figure.

The fact remains though that if you are a company car driver, those official figures do matter as they are what determines how much company car tax you pay. For the same tests also determine the official CO2 figure, on which the Benefit in Kind (BIK) tax rates are based.

What you may not know is that there is a direct correlation between fuel consumption and CO2 emissions - the one is dependent on the other. In fact, both the What Car? and EU tests measure the amount of CO2 emitted from a tailpipe and then convert that figure into fuel consumption.

So if you are lumbered with a lot of BIK tax to pay, you may be grateful that the legislators aren't using the What Car? figures.

Running that Toyota Prius as a company car is a good bet at the moment. Officially, it registers as having 89 g/km of CO2 which translates into a lowly 10% tax liability, about as low as you can pay without having a zero-rated electric car. But the True MPG figures register the real-world CO2 as being an average 132 g/km, theoretically putting it into the 17% tax band. It would be even worse if you're running a 2.0 TDCi Mondeo as a company car and paying 19% tax liability. Our real-world CO2 figure would lumber you with a theoretical 34% tax banding, including the 3% penalty for diesel.

So are there any plans to change the legally required test? There are indications that Europe will revise the system by 2020 and it's likely to include a real-world element, bringing it closer to the What Car? way of doing things. Indeed, transport minister Norman Baker has admitted that a reform of the test is needed.

However, as we all know, any changes at European level have a tendency to get bogged down in years of bureaucracy, and car makers that have spent billions of euros attempting to reduce CO2 and improve fuel consumption - and heavily marketing that they are - could be unwilling to sign up to a new system that makes them all look worse overnight. Not to mention that many individual European countries have CO2 and mpg-based taxation systems that will need unpicking.

That said, some of those very same car makers have already admitted to us that they are unhappy with the current regime and want to engineer their cars to perform better in real-world conditions, rather than just appeasing the regime of the rolling road tests. 'Some cars are being made less efficient than they need to be, which is crazy. We need to change the way we think,' one senior car company source told us.

Whether those same engineers will be focusing on passing the official test or on improving real-world figures, millions of dollars are being spent every day trying to force miles per gallon up and CO2 emissions down. But the headline efforts are being directed towards new technologies for engines, including increasing electrification, be it hybrids, full electric vehicles and so-called range extenders such as the new Vauxhall Ampera.

There is another way to achieve improvements in fuel consumption, and that is to make cars lighter. Because of the pressure to make them bigger, safer and better equipped, cars have been steadily getting porkier. Heavier cars require more energy to accelerate, which increases fuel consumption. That trend is now reversing and you can expect more cars to be made from novel materials, including aluminium, carbon and new plastic technologies in an effort to bring down weight.

This should mean that both fleet drivers and fleet buyers should benefit from lower running costs. But changing technologies will also affect the type of cars that fleet managers choose. Will we be driving more electric cars? Probably not. Except for those who travel only short distances in town and have good access to charging points, these are unlikely to be practical for some time yet.

What is likely is that diesel will no longer be the default choice for company car taxpayers. Why? Because the cost difference between petrol and diesel is closing and will soon be in favour of petrol as engines get ever more efficient. More stringent emissions regulations will also mean that diesel engines will get ever more expensive from 2016.

Many experts also believe that the gap between the pump prices of petrol and diesel will carry on getting wider too. This is largely because of inefficient European fuel refineries that can't cope with the demand for diesel but are still producing surpluses of petrol. If the economy improves, so will the level of commercial diesel use and so this situation will get more acute. This should mean a renaissance for the petrol engine in the company car market.

Fuel economy is a complicated subject but it's also the number one priority of every car-maker on the planet. So even though the price of fuel is going up, our cars will be using less of it. Just remember that you're unlikely ever to match those miles per gallon that you saw quoted in the adverts.



The official fuel tests or the New European Driving Cycle (NEDC) as they are more formally known are conducted on a rolling road in a laboratory.

Two tests are carried out. In the extra-urban test, the car accelerates extremely gently and then settles down to a short cruise. For instance, 0-43 mph takes a glacial 45 seconds and in total the car averages 39 mph.

The other test is the urban test. It takes 195 seconds during which time the car is accelerated to 10, 20 and 30 mph in its three lowest gears. The car is also allowed to idle for a minute. This is repeated four times. The often-quoted combined mpg is derived from an average of the two results.

The advantage of the NEDC test is that it's supremely consistent and offers an easy comparison between new cars. The disadvantage is that it clearly doesn't reflect normal road driving.

By contrast, the What Car? True MPG figures are worked out on British roads and in real traffic. However, like the official tests they use highly sophisticated equipment to measure and analyse every exhaust emission in every car, each of which has two test drivers.

A test route of around 60 miles includes urban driving, averaging 15 mph, followed by extra-urban dual carriageways and motorways at an average of 65 mph, rounded off by another urban loop. The testers drive at a steady pace and avoid aggressive acceleration and braking. 

At the end of the test, the average fuel consumption is calculated, but as traffic volumes and weather vary, this is just a starting point. Sensors attached to the car's ECU system record road speed, exhaust pressure and the throttle throughout the test. Other equipment measures air pressure, altitude and humidity at any given moment. These are factored in to produce an accurate True MPG test.

To find out more go to




True mpg                            

Official mpg                               

BIK tax rate                    'Real World' BIK        
Mercedes S350 CDI £59,195 37.3 36.7 29% 35%
Jaguar XK
Convertible V8
£71,430 24.9 25.2 35% 35*
Audi A4 2.0 TDIe £27,010 54.3 65.7 16% 21%
Kia Sportage
1.6 GDI 1
£16,515 40.7 44.1 20% 24%
Toyota Avensis
2.2D T4 Auto
£25,640 43.3 45.6 28% 30%

* (already maximum tax band)




True mpg                            

Official mpg                               

BIK tax rate                     'Real World' BIK        
Peugeot 3008
£27,779 46.0 70.6 14% 28%
Ford Focus 1.6
TDCi 115 Titanium
£19,965 43.1 67.3 15% 30%
Renault Megane
1.5 DCI GT
£21,300 45.8 80.7 14% 28%
Toyota Prius
£24,130 52.2 70.6 10% 17%
VW Golf Bluemotion
SE 1.6TDi
£17,731 51.8 68.9 16% 21%

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