Fuel Efficiency Tests
Every car produced today is tested to try and determine accurate fuel efficiency data. Each car is analysed on a machine called a dynamometer, which is like a large treadmill for a car. While the engine and transmission drive the wheels the vehicle never actually moves - just the rollers upon which the wheels are placed. A professional driver runs the vehicle through two standard driving schedules, one each to simulate city and highway driving conditions, and ensures he or she is maintaining the correct pace via a real time computer display.The "city" program is designed to replicate an urban rush hour driving experience in which the car is started with the engine cold and is driven in stop-and-go traffic with frequent idling. The "highway" program, on the other hand, is created to emulate rural and high speed driving. It is performed with a warm engine and includes fewer or no stops (both of which contribute to better fuel economy). Both fuel efficiency tests are performed with the cars air conditioning and other accessories turned off.
Throughout the tests, a hose is connected to the rear of the vehicle to collect the engine's exhaust fumes. The amount of carbon present in this mixture is measured to calculate the exact amount of fuel burned. This is considered more accurate than using a fuel gauge to physically measure the amount being burned. The final fuel efficiency figures are adjusted downward to help reflect the differences between what happens in a lab and out on an actual road.
Fuel Efficiency - Real World versus Published Figures
A host of physical and personal factors contribute to the differences between a vehicles rated and real world energy consumption. For starters, cars used for evaluation in the fuel efficiency tests are "broken in" and are in top mechanical shape. New vehicles don't usually attain their peak mileage until they're driven about 3,000 to 5,000 miles, and ill-maintained vehicles will consume far more fuel than those that are in perfect condition. Even relatively minor upkeep factors like having incorrect air pressure in the tyres can vastly affect your vehicles fuel economy.Depending on where you live, the particular blend of petrol or diesel sold in your area at a given time of the year may have more or less energy content, which in turn results in better or worse fuel efficiency. What's more, even small differences in manufacturing and assembling can cause minor disparities from one otherwise alike model to another.
Also, the cars and trucks subjected to official testing are driven without a full complement of passengers, cargo, and options aboard - all else being equal, the heavier a vehicle is, the more fuel an engine will need to burn. Similarly, the vehicles are tested without the air conditioning and other electrical accessories in use, which lessens the load on the engine.
Other physical factors like trip length, traffic conditions, terrain, temperature, and the weather all affect your mileage. Likewise, installing exterior accessories like roof racks and cargo carriers that hamper a vehicles aerodynamics will take their toll at the pump - the more aerodynamic drag that's placed on a vehicle, the more energy it takes to run it, especially at higher speeds. Lead-footed acceleration, heavy braking, fast driving, excessive idling, towing, and engaging four-wheel drive will also drain your vehicles tank at a higher rate.
Published fuel economy figures may not be a completely accurate prediction of the kind of mileage you'll register but it's still valid as a source of comparison when you shop for a new car.
For more information on lowering your fuel bills check out our practical tips to improve fuel economy.
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