What happens if... you drive in 5th gear at a speed of 55 km/h

Any car has its own mass, and energy is required to accelerate it to a certain speed. Moreover, the more intense the acceleration, the more energy needs to be expended. This is known to everyone. It is also no secret that with uniform motion, fuel is consumed less, but here a lot depends on aerodynamic drag, which grows in proportion to the square of speed and is directed against the movement. But the power needed to overcome this force increases already in proportion to the cube of speed. In other words, if a car accelerates from 50 to 100 km/h (that is, twice), then the air resistance that has to be overcome increases not by two, but by as much as eight times! More precisely, to maintain such a speed, 8 times more energy will be required. It is on "pushing apart" the air that those very horsepower are spent, which at 50 km/h need about 30, and at 120 km/h — already 130.

To make the picture clearer, it is enough to look at the graph, which, using the example of a ZAZ car, demonstrates how power is lost with increasing speed.

In addition, it is worth remembering that engines achieve maximum efficiency in the zone of peak torque and at minimum rpm. It follows that the most economical mode is driving in the highest gear at the lowest possible speed. However, depending on the type of engine and gear ratios of the gearbox, the optimal speed in the highest gear may differ for different cars.

In theory, it is quite possible that by moving in fifth gear (provided there are only five), you can achieve the lowest fuel consumption. But why then do almost all automakers in their operating instructions recommend driving on the highway in the highest gear at a speed of 80–100 km/h, and not 50–60 km/h?

Opinion. Mine and wrong

Every person has the right to their own opinion on any issue. And about which gear and at what speed it is better to drive, almost all drivers know, as it seems to them. But in order to resolve this dispute, it is better to turn to professionals, so Autoportal asked the official Nissan dealer in Kyiv — the company "KIY AVTO" to comment on the situation.

"If we talk about driving at a speed of 55 km/h in the highest gear, then you need to clearly understand: a two-liter engine in a car weighing two tons in this mode will indeed spin at low rpm. But due to the large weight of the car, it will be under maximum load. As a result, due to low oil pressure in the cylinder block, wear of parts will be maximum. Therefore, to avoid excessive wear, a lower gear is more suitable for a speed of 55 km/h. At the same time, the same two-liter unit, but installed on a light hatchback, will cope with the load more easily due to the smaller mass of the car, however, for it, it will also be more critical than, say, in fourth gear. I believe that you should not get carried away with driving in higher gears at low speed. Yes, the lower the rpm, the lower the consumption, but at low rpm in the highest gear, the engine suffers greatly and can fail. So here the question of priority is: either save 50 hryvnias on fuel, or then pay 5000 hryvnias for engine repair," — believes Sergey Kuzmichev, Nissan car sales specialist.

Thus, we can conclude that when driving in fifth gear at a speed of 55 km/h, it is indeed possible to achieve minimal fuel consumption, but at the same time there is a high risk of damaging the engine or its individual components.

How to become a fuel economy record holder

Best of all, Andrey Sidorenko, automotive journalist, author of the National Fuel Economy Record will be able to answer this question:

"You need to look at the characteristics of a particular engine. For example, on a Skoda Fabia with a 1.4 engine, you can safely drive in fifth gear at 2000 rpm and a speed of 60 km/h. This mode will save fuel without harming the engine. But it all depends on the road profile (uphills and downhills) and the degree of vehicle load. I also want to dispel one of the misconceptions regarding driving with cruise control on, when supposedly the car itself chooses the most economical mode. This is only true for flat horizontal sections."

Let us remind you that a year ago, it was Andrey Sidorenko together with Gennady Mazepa who drove a 2-liter Subaru Forester 489.6 km with an average consumption of 5.3 liters, which is 24.6% better than factory indicators. The record holders are in no hurry to reveal all their secrets, but they claim that only the ability to smoothly press the gas pedal and brake less often, moving at an optimal speed in the highest gear, is not enough.

According to them, the main thing is to maintain the inertia of movement. Therefore, first you need to learn how to brake correctly with the engine and take turns so as to lose speed minimally. This is exactly what counter-emergency training is for, the specialist in which was Gennady Mazepa. He believes that fuel economy records are within everyone's power, but for this you need to carefully monitor the engine rpm:

"The number of revolutions plays a key role. At a speed of 55 km/h, on almost all passenger cars, the rpm will be around 1.2–1.4 thousand. The consumption will indeed be very small, but this is actually killing the parts of the crank mechanism. Low fuel consumption during normal engine operation is usually achieved in the range of 1800–2000 rpm. For diesel and turbocharged engines, this indicator may shift in one direction or another depending on design features."

How to drive economically and without harm to the engine

The company "Eurocar", which represents the Skoda brand, is one of the few that regularly held eco-rallies, that is, competitions where the participant who completed the route with the lowest fuel consumption wins. The organizers determined the winners not just by actual consumption and engine size, but used the so-called Fuel Efficiency Index (FEI), which is the arithmetic mean between real fuel consumption and consumption per ton of vehicle weight.

"Eurocar" held these competitions among car owners for three years. As far as we know from a survey of participants, many during the rally used such well-known techniques as maximally smooth driving without sharp accelerations and braking, coasting, and some even turned off the engine at traffic lights and in traffic jams. When driving with maximum efficiency, it is important to choose the right gear for a specific speed and get into the desired rpm range (this depends on the engine characteristics). You should always follow the manufacturer's recommendations. Now in any car operating manual there are sections devoted to economical driving," — reported to Autoportal in the press service of LLC "Eurocar".

From all the above, it follows that before you start saving by driving at a speed of 55 km/h in the highest gear, it is worth studying the sections "Technical characteristics" and "Economical driving" in the instructions. In most cases, it describes in detail what rpm is optimal and which gear is better to choose for a particular speed.

Determining engine load using the old-fashioned method

If you never had an operating manual, and the tachometer in the car is not provided, then to assess the engine load when driving at a speed of 55 km/h in fifth gear, you can use the old proven method. In this matter, it is worth listening to the opinion of engine repair specialists:

"You have probably noticed that taxi drivers know best about driving in the highest gear. They do not even have time to accelerate properly, and they already switch to fifth. But an experienced taxi driver differs from a beginner in that he listens to the engine and pays attention to detonation. If it appears, he shifts to a lower gear, gains speed, and again engages a higher gear. Here is the main recommendation for those who want to save money without risking "killing" the engine," — believes engine repair specialist Anatoly Demchenko.

Basic rules

- Driving in fifth gear at a speed of 55 km/h can be safe for the engine and economical if the engine speed is in the range of 1800–2000 rpm

- If there is no tachometer, you need to focus on vibration and detonation. If they are present, the engine is operating under a large load, and you need to shift to a lower gear, and then again engage a higher gear

- Each car has its own optimal speed for minimal fuel consumption in the highest gear

- Engaging a higher gear too early during operation can lead to a quick visit to an engine repair specialist

- For driving at a speed of 55 km/h in fifth gear, cars (mostly small cars) with shorter gear ratios in the gearbox are best suited, where the rpm at 100 km/h is about 2500–3000 rpm, and at 55–60 km/h — up to 2000 rpm

- It is better to study the operating manual in detail about the optimal speed in the highest gear for diesel and turbocharged gasoline engines. The features of their operation differ significantly from naturally aspirated gasoline engines.

It is worth noting that the engine repair specialist we interviewed expressed the opinion that fuel consumption can be reduced by regularly washing the engine compartment. According to him, a clean engine consumes up to 5% less fuel. However, none of the economical driving experts and official dealer representatives we interviewed confirmed this opinion, claiming that a clean radiator, serviceable spark plugs, and cooling system are much more important. Have you noticed a decrease in consumption after washing the engine, and at what speed, gear, and rpm does your car consume the least fuel — write in the comments and on the Autoportal page on Facebook, indicating the make, model, and engine of the car...