How to find out your car's real mpg figure

Official fuel economy figures aren't up to much. But there's a much better way of finding out your car's real mpg figure

Dominic Tobin
May 18, 2018

If you want to know how much a car will cost you in fuel, then you should completely ignore the official mpg figures that are published in every car brochure and advertisement.

At best misleading, these numbers often bear very little resemblance to your car's fuel economy in real-world conditions on public roads.

That's because they are generated from a European-standard laboratory test. Every car undergoes the same process and manufacturers must publish the figures by law, which is meant to give buyers a fair way to compare diffrent models.

Despite recent improvements, the test still makes most cars appear more efficient than they are on real roads and in proper traffic. Manufacturers have also become increasingly good at tweaking their cars and procedures to boost the official mpg figure, creating the sizeable disparity between the results and fuel economy that drivers see.

That’s where real-world testing comes in.

 

Real-world mpg testing

To give drivers a more accurate estimate of the fuel economy that they can expect - and the amount that a car will cost to run - sophisticated testing equipment is strapped to the exhaust of cars, which are then driven on a set route, taking in different roads, speeds and gradients.

It measures what comes out of the exhaust pipe and the amount of fuel being used. At the end of the test, sophisticated software adjusts the data to take into account the temperature and traffic conditions of each run - allowing the data to be compared with real-world tests of other cars. Scroll down for more information on how you can view these figures in the Equa Index,

   

Real-world mpg figures from the Equa Index

Since last year, data collected by Emissions Analytics, a testing company, has been published in a table called the Equa Index.

The firm has tested more than 800 cars on the road (above), and says that it has enough data to accurately predict an average mpg figure for most current cars. These estimates are also included in the Equa Index.

Data from the index is being incorporated into advice and buying guides on BuyaCar, but you can also check the real-world mpg figure for any model by checking the full index.

You can see whether the model you’re interested has been tested, because it will have a letter T in the final column of its entry. Estimated mpg figures are shown with the letter F - for forecast.

     

Change your driving style to improve real-world mpg

The chances are that your car will be less efficient than official figures suggest, but you can claw back some savings by driving it smoothly; aggressive and inefficient driving can add hundreds of pounds to your annual fuel bill.

A survey at the beginning of 2017 found that drivers with the calmest driving styles spent on average £837 a year on fuel, compared with the most aggressive motorists who paid out £1,399.

The difference of £562 each year - or almost £50 a month was uncovered by Direct Line insurance after an analysis of real journeys made by policyholders in cars fitted with a black box, which relays information about their driving style to the company.

Accelerating gently, slowing down gradually and changing up gears early are among the techniques that help make journeys smoother and reduce fuel use.

Other studies have also found that driving styles can make a big difference to fuel consumption. A report from the RAC Foundation motoring research charity on the effectiveness of Eco Driving, found that aggressive driving can increase fuel consumption by 37% and identified a series of techniques to boost fuel economy, outlined below,

  • Tyre pressure Tyres inflated at the correct pressure minimise resistance as they roll down the road, helping to maximise your car’s efficiency.
  • Windows Using air conditioning with the windows closed reduces wind buffeting at high speeds, making your journey quieter and more fuel efficient. When driving at less than 40mph, it’s more efficient to switch off air conditioning - which uses energy from the engine - and to open the windows.
  • Avoid short trips The first few miles of a journey can be the least efficient, because your car won’t have warmed up. Cold engines use more fuel than warm ones.
  • Avoid idling your engine If your car has a start/stop system, which automatically turns the engine off when you’re waiting in traffic, then leave it on. Stopping and restarting the engine uses virtually no fuel, and you’ll save for all of the time that it’s not running.
  • Smooth driving The most effective way of cutting your car’s fuel consumption is to drive smoothly. It’s particularly effective when you’re speeding or slowing down. Press the accelerator as gently as possible and coast to red traffic lights or junctions - letting the car roll along and slow down on its own without pressing the brakes - and your fuel consumption should soar.
  • Change gear early Don’t wait until the engine’s revving hard before you change up: if you shift as soon as your car can cope with the next gear, then you’re likely to reduce fuel use - as long as you don’t mind slower performance.

      

Boost real-world mpg with fuel-saving technology

  • Stop/start It’s unusual to see a new car without a stop/start system. These automatically shut off the engine when you come to a halt - at traffic lights, for example - to save fuel when the car isn’t moving.
  • Advanced sat-nav Manufacturers including BMW, Audi and Mercedes have fuel-saving information linked to their more advanced sat-navs. As you approach roundabouts or speed limit changes, you’ll see a symbol of a foot lifting off the accelerator. If you do as the car suggests, and allow it to coast along, slowing down gently, it should be timed perfectly, so you’re at exactly the right speed when the junction or new limit arrives. It will irritate anyone behind you, though.
  • Eco gauge For driving that feels more like an exam, turn on your car’s eco gauge if it has one. You’ll then be scored on the efficiency of your driving. Mercedes’ system, for example, grades you in three different areas: your acceleration, driving smoothness and the amount of time that you coast - keeping the car moving without pressing the accelerator and brake. Nissan has a similar system (above), while Honda displays virtual leaves on the dashboard - the more that are displayed, the more efficient your driving.
  • Cylinder deactivation You need a powerful engine for brisk acceleration when you’re driving away from a standstill or pulling out to overtake, but much of this is unused when you’re simply driving along at a steady speed, so some cars can shut down part of the engine when it’s not needed to save fuel. As soon as you press the accelerator, the engine returns to full power. The transfer is so quick and smooth that it’s rarely noticeable. The technology is known as cylinder deactivation (or cylinder-on-demand) because engines are made up of a number of cylinders within which fuel is burned. When part of the engine is shut down, fuel is no longer pumped to one or more of these cylinders.

      

Real-world mpg of hybrid cars

Working out the fuel economy you can expect from hybrid cars - particularly plug-in versions - can be difficult because they operate best in certain traffic conditions. The amount of fuel that you use will depend largely on where it’s driven and for how long.

That’s because hybrid cars work best in stop-start traffic where fuel is ordinarily wasted. Hybrid cars are able to recover energy that’s usually lost during braking and use it to charge up their batteries. When you accelerate, the batteries drive a motor that boosts power and reduces the need to rev the engine inefficiently.

In town, the fuel economy difference between a non-hybrid and hybrid can be 20mpg or more. But if you’re just making a long motorway journey, the savings are likely to be smaller or non-existent - particularly when compared with a diesel car.

Fuel economy for plug-in hybrids will vary even more, making their official mpg figures virtually useless. After being plugged in to fully charge their batteries, these cars can travel for several miles on electric power alone (although it won’t surprise you to learn that official claims for their electric range are usually over-optimistic). If you regularly make short journeys, then you may use no fuel at all.

But once the batteries run low, plug-in hybrids use petrol or diesel power. The longer you drive with the engine running, the worse your fuel economy will be. This is particularly the case for big plug-in hybrids with petrol engines, such as the Volvo XC90 T8. On motorways, with the petrol engine running, you’ll use much more fuel than a standard diesel model.

   

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