The less we drive, the less we consume? That’s true… but only above a certain speed. Below, the opposite phenomenon applies. We explain why and above all, how many km/h you need to drive to really save fuel.
Faced with the spectacular rise in gasoline prices at the start of spring, motorists are looking to save money by any means possible. Find the cheapest service station, reduce journeys, favor carpooling, or even adopt eco-driving gestures… This is what we are taught at driving school: well-inflated tires reduce fuel consumption, as does a less loaded car, moderate use of air conditioning or heating, but also and above all smooth driving, where braking is anticipated and gears are shifted at the right time to avoid accidents. energy-consuming over-diets. In short, practicing eco-driving would save 5 tanks per year, according to Ademe estimates. But most of us think that this also means reducing the overall pace. Yes, it’s a known fact: the less you drive, the less you consume. However, this is not entirely accurate.
This is indeed the case on high-speed journeys, such as on the motorway or other expressways for example. Above 80 km/h, air resistance increases and the engine therefore devotes a large part of its energy to fighting against the wind, which exponentially increases gasoline consumption. Driving 10 km/h below the authorized speed limit therefore allows you to make significant savings: driving at 80 km/h instead of 90 km/h allows you to save 120 euros in fuel per year, according to Road Safety calculations (and it is surely even more since the price increase!). Driving at 120 km/h instead of 130 km/h represents a saving of around 20% in fuel, for a paltry loss of time of only a few minutes. However, this principle ceases to be true when we talk about low or medium speeds, applied in all urban areas for example: below 80 km/h, the slower we drive… the more we consume!
Imagine having to travel 100 km: at a speed of 20 km/h, the journey will take you 5 hours. These are therefore 5 hours during which the engine runs continuously. But by driving at 60 km/h for example, you only need 1 hour and 40 minutes to make this same journey. The engine therefore runs for much less time, and consumes significantly less. Especially since on manual gearboxes, the gear ratio plays a major role: on average, a car consumes 15 liters of fuel per 100 km in 1eraand the figure decreases with each passing speed. In 4th speed is already half as much, with 7 liters per 100 km. Concretely, consumption gradually decreases as the pace increases. But after 80 km/h, because of this air resistance which forces the engine to make a greater effort to move forward, consumption starts to rise again. The perfect balance point, which experts call the “sweet spot”, would therefore be around 70 km/h: this is the exact speed at which we spend the least fuel.
Of course, let us remember that respecting the Highway Code and safety come before the simple financial question, and it is therefore appropriate to respect the speed limits. On the other hand, if the situation allows it, it is better to stay between 60 and 80 km/h rather than accelerate, both to lighten the wallet and to reduce the risk of accidents, and also reduce pollution! Moreover, the United Kingdom has understood this well: with the rise in gasoline prices and the risk of a shortage due to the war, the country is considering applying a temporary speed limit of 80 km/h throughout its territory to deal with the situation.
