Can you match your car’s publicised mpg?

It’s often cited in these pages that the rising cost of petrol and diesel means fuel economy is increasingly becoming a deciding factor when choosing a new car. In response, manufacturers are bandying about consumption figures of over 80 miles-per-gallon (mpg) – but just how realistic is it for a ‘normal’ driver to match a car’s published mpg? The seeds of doubt were first sown back in March, when a study by lobby group Transport & Environment (T&E) suggested car manufacturers were manipulating fuel consumption and CO2 emission tests to bump up cars’ performance figures by an average of 23%.


So let’s say you cover around 12,000 miles each year, you’re in the market for a fuel efficient car and plump for a particular model on the back of its promise to return a whopping 83mpg. This means you could expect to pay £936.60 in fuel, based upon the average price of a litre of diesel at 142.5p, as it is today. However, if the figures have been bumped up by 23%, this means the car may actually only be capable of 63mpg and so same annual mileage will cost £1,233.94 – which represents a completely unexpected increase of £297.34. So is it possible to get anywhere near your car’s estimated mpg? To try and answer this, we first need to look at how manufacturers test their vehicles.

The fuel-efficient coefficient

When you’re studying a car’s fuel economy, you’ll be given three figures under the headings Urban, Extra-Urban and Combined. To get the mpg figures for each category, cars are not put through their paces on the UK’s highways and byways, as you might reasonably expect, but simply placed on a rolling road. To work out Urban mpg, a car is made to accelerate and decelerate several times, ‘drive’ at a steady speed and idle over the course of a 2.5 mile (yes, a two-and-a-half mile) drive. The maximum speed in this test is 31mph, the average speed is 9mph and the acceleration is so slow that it takes 26 seconds to hit that magic 31mph. Extra-Urban is carried out under the same conditions, with 50% driving at a steady speed, a top speed of 75mph (it’s anyone’s guess how long it takes to hit that speed) and an average speed of 39mph. The figure for Combined is worked out by taking an average figure from the previous two results, based upon the distance covered. As you can see, the results are not going to be a reflection of real driving conditions, with no headwinds or hills to add to consumption figures. And that’s before you even consider that the testers might pump up the tyres extra hard to reduce rolling resistance, tape up grilles and remove wing mirrors to reduce resistance and disconnect brakes to ease friction. While it seems some car manufacturers are employing tactics that will increase fuel efficiency figures, questions also need to be asked of the New European Driving Cycle (NEDC), which sets the criteria for these tests and dictates these are the only figures manufacturers are allowed to quote. So, time for the $64million question…

Can you hit your car’s estimated mpg?

Probably not, is the short answer. I set myself the challenge of doing just this last month when I took a Chevrolet Volt for a week-long test drive. Admittedly, I was setting myself a pretty big challenge as official Chevrolet figures quote that the Volt can do some 235.4mpg (no, really), and although I worked out I could probably get a pretty exceptional 100mpg out of it with some considered driving and regular recharging of its lithium-ion battery, this was still nowhere near the manufacturer’s claims. And although you may not be able hit your car’s estimated mpg, there are some simple ways you can up your car’s efficiency. Have you ever seen a big discrepancy between your actual fuel consumption and the numbers quoted in your car’s promotional material? Would fuel costs affect your decision to purchase a particular model? Join the debate in the box below or on Twitter using the hashtag #FocusonFuel

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