Drive 10,000 miles a year in a Mazda3 1.6 and, at current prices, it'll cost you £2500 in fuel over three years. Drive the same distance in a Ford Focus 1.6 – which offers much the same performance and accommodation as the Mazda, and costs about the same – and you'll have to spend nearly £500 more in fuel because the Focus is less economical. And that's if you drive sensibly: drive badly and you can easily burn 50% more fuel.

The difference between an economical car driven well and a less efficient machine driven insensitively could easily amount to £1000 a year – plus the time and inconvenience of having to fill up twice as often. Fuel is the most obvious cost of running a car, the most frequent and the most visible bill you have to pay to stay mobile. And yet few people do much to minimise their fuel costs, despite the big savings to be had by choosing a car wisely and driving it with economy in mind. The further you drive and the most expensive fuel gets, the more you'll save.

Choosing an economical car is made easier thanks to Government legislation introduced in the 1970s which required manufacturers to supply fuel consumption figures for every new model, and laid down strict testing regimes to ensure that the figures were comparable for all cars. Figures were published for urban driving and constant speeds of 56mph and 75mph, given in miles per gallon or mpg. Today those tests have been refined to give us three different fuel consumption measurements, called the Urban Cycle, Extra-Urban Cycle and Combined figures, set out in EU Directive 93/116.

But just how are these different fuel consumption figures arrived at, what do they mean, and how can they be used to choose an economical car?

The tests explained

All the tests are carried out in a laboratory, either by an independent testing organisation or by the manufacturer itself. All the test labs are inspected and approved, and some of the tests are independently witnessed to ensure fairness. The test cycles themselves are the same as those used to measure the official CO2 emissions figures which are now used for vehicle taxation.

The Urban Cycle fuel consumption figure, as its name suggests, represents fuel consumption in town driving and consists of a sequence of stop-start and steady-speed motoring from a cold start with a maximum speed of 31mph and an average speed of 12mph. The test covers 2.5 miles.

The Extra-Urban figure represents out-of-town driving. About half of its 4.3-mile length is covered at steady speed, the rest being acceleration, braking and idling.

The Combined figure is the fuel consumption over the whole 6.8 miles of the Urban and Extra-Urban tests combined together, giving an average figure over a wide range of driving conditions.

Where these three official figures help is that they make it easy to compare different cars, knowing that each one has been tested in the same way. By law all new cars in showrooms must carry a label detailing their official fuel consumption figures, and dealers must be able to supply customers with fuel consumption data on request or face a fine of up to £5000. Adverts and sales brochures which refer to fuel consumption must also quote all three fuel consumption figures both in miles per gallon and in litres per kilometre (which is more commonly used in Europe).

Choosing an economical car

If you spend most of your time driving in town, pay particular attention to the Urban Cycle figure. If most of your motoring avoids town-centre jams, the Extra-Urban figure will probably be more important. The Combined figure is more useful where you expect the car to be used in a wide variety of locations and driving conditions, from traffic jams to the open road, and it’s the most widely quoted. This is the figure you'll find in the Fuel Economy column of Carbuyer's new car data section.

Hidden in the lists of fuel consumption data are some general rules for choosing a car with good fuel economy. Don’t buy a car that's bigger than you need, for instance: a heavier car uses more fuel each time it accelerates, and a physically bigger car burns up more fuel because it has to push more air out of the way as it moves. Both factors are particularly true of 4x4s, which are built heavy for strength and tall for practicality. So don't buy an off-roader unless you intend to drive it off-road.

Generally you should avoid automatic transmissions if you want to achieve the best economy. Autos are much more efficient than they used to be, but even now most of them will give a car poorer fuel consumption than a manual gearbox. Tall gearing, on the other hand, helps lower engine revs during motorway cruising and can help to reduce fuel consumption as well as engine noise. If you plan on using the car for lots of motorway miles, scan the spec sheet for 'mph per 1000rpm' in top gear: the bigger the number the better.

At the other end of the scale, hybrids like Toyota's Prius and Honda's Civic IMA and Insight are more economical than most conventional petrol-engined cars in urban conditions. Hybrids are powered by petrol engines but also have an electric motor which can assist the engine during acceleration. The petrol engine is automatically switched off when the car is stationary in traffic.

In the real world

While the official figures are helpful for comparing cars, they don't necessarily predict accurately the fuel consumption you will achieve, for several reasons.

First, cars are mass-produced and no matter how effective the manufacturer's quality control systems are, each one is slightly different. Two cars of exactly the same specification can produce different fuel consumption figures simply through minute differences in their construction.

Second, the cars are generally tested when still new: the rules say only that they must be run-in and must have covered at least 1800 miles. When a car gets older it may not be as efficient or as consistent in its fuel consumption, and the deterioration will be more marked if servicing is skimped.

Weather and traffic conditions also play a huge part in determining fuel consumption. The official tests are carried out in a lab, where conditions are 'perfect'. Out on the road you might encounter headwinds, heavy traffic, roadworks, delays and all manner of other irritations which do your blood pressure – and your car’s fuel consumption – no good at all.

But the biggest influence on fuel consumption is the way you drive. Harsh acceleration, poor observation and driving in the wrong gear all make a big impact on the amount of fuel your car uses. See our box on ‘Driving for Economy’ for tips on how to save fuel.

Testing the Tests

The official tests are fine for comparing cars, because every car is tested in exactly the same way. But do the figures bear any resemblance to those you can expect to return in the real world? We decided to find out, by comparing the official figures with our own set of fuel consumption measurements.

The car we used was an Audi A3 with the 2.0FSI petrol engine. The Audi's clever FSI direct-injection system – which has much in common with diesel engine technology – gives it the potential for excellent fuel economy, but also good performance thanks to a maximum power output of 150ps.

The Audi's official Urban figure is 29.4mpg. In very heavy traffic we recorded less than 20mpg, but away from rush-hour congestion (though still in town) the Audi returned up to 35.9mpg. So the official figure is near the middle of our range of results. We'd expect most A3 2.0FSI drivers to return around 30mpg in town driving.

In our real-world version of the Extra-Urban test, we recorded fuel consumption figures which varied widely. Using all the Audi's performance on the open road meant we could only travel 24 miles on every gallon, but keeping to the official Extra-Urban test's average speed of 39mph and driving with economy in mind the A3 produced an excellent 62.4mpg. The Audi's official Extra-Urban figure is 53.3mpg, which is certainly achievable on the road – though we expect most Audi drivers would be inclined to use the performance available, and that would probably result in higher fuel consumption. Even so, 45mpg should be easily achievable most of the time, without dawdling.

The official Combined figure, a weighted average of the Urban and Extra-Urban numbers, is 38.2mpg. Our best results give a combined figure of 49.2mpg. So the official figures can be bettered, but as we've said elsewhere, insensitive driving and traffic jams will quickly cause the figure to drop.

Drive for Economy

  • Accelerate gently Acceleration uses lots of fuel, and the harder you accelerate the more fuel you burn. So gather speed gently rather than racing away. And there's no point accelerating hard from one traffic light, then braking hard to stop at the next one just up the road – it just wastes fuel.
  • Change up early Higher gears save fuel, so change up to the next gear as soon as possible. In most cars you can change up at 2000rpm without problems, but every car is different. In an automatic, use the D position and a light throttle opening to let the gearbox choose the right gear.
  • Anticipate road conditions Leave a big gap between your car and the one in front. When traffic ahead slows down, don't immediately jump on the brakes. Instead, back off and let the gap close up. Then accelerate gently as the car ahead pulls away. If you can avoid using the brakes by thinking ahead, you'll save fuel and be a safer, smoother driver.
  • Avoid high speeds Fuel consumption rises rapidly the faster you go, so if you're trying to keep your fuel bills down, keep your speed down. But don't drive through the town instead of using the bypass on the grounds that the speeds will be slower: a gentle cruise will always use less fuel than a stop-start journey.
  • Avoid short trips All cars use more fuel when the engine is cold. According to Audi, a medium-size car will take 2-3 miles for the engine to warm up properly, during which it will achieve just 8mpg – and emissions will be high. So short journeys are bad for fuel consumption, and bad for the environment.
  • Switch off in traffic Stationary in a traffic jam your car does 0mpg. If you are going to be static for more than a minute or so, switch off the engine to save fuel.
  • Use your trip computer If your car has an on-board computer, check to see if it can provide an instantaneous mpg function. Keep an eye on this as you drive, and you'll soon learn how different driving styles use more or less fuel.
  • Keep windows closed Driving along with the window open upsets the air flow around the car, causing what engineers call 'aerodynamic drag'. The car has to work harder to push the air out of the way as it moves, and that means more fuel is used – particularly when cruising at high speed. So keep windows and sunroofs closed. A roof rack will also create extra drag, even when empty – so remove it when you're not using it.
  • Avoid unnecessary weight Carrying extra weight means you use up more fuel when accelerating, so clear out the junk in the boot and leave the dog at home. If you use your car in town most of the time and you're not worried by the inconvenience of stopping for fuel more often, fill the tank half-full each time to save weight.
  • Keep tyre pressures correct A well-maintained car will be more efficient, so make sure your car gets serviced on time. Something you can do yourself is to keep your tyre pressures up to the mark. Soft tyres flex more as they roll, generating more 'rolling resistance' – and to overcome that the engine burns more fuel.
  • Use electrics sparingly Your car battery is recharged using an alternator driven by the engine, so you pay for using electrical items in greater fuel consumption. Switch off heated screens as soon as you can, and if you have air conditioning fitted, use it sparingly – and use the 'economy' mode if there is one.

Published in CarBuyer magazine, 2004