A given hybrid for less than 6 liters of super per hundred kilometers? A beautiful Diesel SUV for less than seven? If only to approach the official consumption values printed on advertising catalogs, you have to travel the roads of a decidedly very flat country and comply with the strictest respect for the precepts of eco-driving. Do not see in this gap between real and official consumption any trick on the part of the manufacturers: they are only taking advantage of the leniency of European regulations.
Over the years, in fact, engineers have devised a thousand ways – all perfectly legal – to optimize the fuel consumption and polluting emissions of the vehicles they present for homologation tests. The values recorded in for the new model presented are valid for each of the thousands of copies produced identically.
Since September 2017, however, the rules of the game have been tightened considerably. Firstly, engines are tested harder, in order to get as close as possible to the reality of driving. On the other hand, each of the variants of the same model must be subjected to two test protocols: the first on a laboratory bench, where the movement of the road is simulated by the rotation of rollers under the wheels of the immobilized vehicle; the second outdoors, when the moving vehicle is equipped with miniaturized measuring instruments. In either case, the technician behind the wheel is responsible for reproducing a codified sequence of very precise accelerations and decelerations. On his control screen, he observes the movements of a cursor which moves according to the pressure of his foot on the accelerator: it is up to him to follow a predefined curve as closely as possible.
Until 2018, the official consumption was that of the lightest model
The importance of the gap between official and actual consumption is also explained by the fact that the manufacturer was authorized until the fall of 2018 to submit the homologation tests to the the most stripped-down variant of its models. The consumer, for his part, almost always buys a car that is better equipped – and therefore heavier – and fitted with wider tires that consume more energy, because it offers greater resistance to forward movement. This practice ended in September 2018 with the entry into force of the WLTP regulation, which requires manufacturers to submit to homologation tests each variant of their models. A painstaking job.
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The Association of European Automobile Manufacturers (ACEA) indicates that the fitting of 18-inch rather than 17-inch wheels results in a increase of 2 g / km of the average CO2 emission value. The penalty is still 1 g / km of CO2 for vehicles which do without a spare wheel and run with four puncture-proof tires (known as “run flat” or “run flat”). This is how each piece of comfort or safety equipment weighs on the scale and inevitably increases fuel consumption.
Large wheels and wide tires increase fuel consumption
This is why manufacturers encourage their customers to go through the accessories box rather than options: the idea is to order a rather stripped-down variant of your sedan or SUV, then entrust your dealer with the assembly of the coveted large wheels, roof bars that increase aerodynamic drag or even the motorized hitch that weighs down the car.