How to know and calculate your car real fuel consumption

The actual consumption of the vehicle differs from that announced in the technical data sheet or dashboard display, being influenced by the degree of traffic congestion, driving style, and other things related to technical problems that the car has.

Fuel consumption and methods of reducing it are always topical. The chemical energy found in the fuel is transformed by the internal combustion engine into mechanical energy, used for moving the vehicle.

Different losses occur during the transformation process so that not all the energy contained in the fuel will be available as mechanical energy. As a result, efficiency is subunit and varies depending on the engine operating regime. Idling is an example of zero efficiency because fuel consumption does not produce useful energy.

Displaying fuel consumption on the dashboard

Most cars today have dashboard computers, which, among other things, show us the fuel consumption we have. Both the instant and the average, and most drivers tend to consider the consumption shown by the computer, instead of the real one. Is there any difference?

Almost all cars now have a function that shows you the average or instant fuel consumption on the dashboard. A useful feature to know when and how our car manages to have the most efficient way of driving.

For example, if after a road with sharp accelerations and aggressive braking, we find that we have high average consumption, it is good to know that next time we approach the same route is best to drive more slowly and carefully if we care about the car’s fuel consumption.

How does instant consumption work?

The dashboard display shows us how much fuel the car consumes in each second. These indices differ depending on the area in which you live. In Europe, this is calculated with liters / 100km, and in the UK and USA with mpg or miles per gallon, although there is a different quantity calculated in those two geographical areas.

On the dashboard, we can see values ​​between 0 liters / 100km and 99 liters / 100km. Instantaneous consumption is calculated by a computer that takes into account several values, such as engine speed, fuel flow, pressure in the intake manifold and throttle position. These values ​​are read by sensors and probes and the information is taken from a computer that displays the value on the display in the car’s cabin.

The average consumption of a car is also calculated, but the value changes on the display less often, either after a certain distance or after a certain time.

What does the computer not take into account when calculating average fuel consumption?

The engine has, from time to time, overloads that the computer does not take into account when calculating consumption. For example, turning on the air conditioner. Or when the car is running slowly, or when it is going fast, and at high speeds due to friction with the air it has to put more effort. There are situations where the consumption increases, but the computer does not have to calculate them.

What is the difference from the real consumption?

The average consumption shown in the dashboard is always lower than the actual fuel consumption of the car. Due to the above reasons, because the consumption displayed by the car is calculated, and the computer cannot take into account all the parameters and all the variables. However, the differences are not very big. For example, someone who can make a real consumption of the car will notice that in a full tank the difference is about 1-2 liters (0.2 – 0.5 gallons). In other words, the consumption displayed by the dashboard is about 0.2 liters / 100 km.

Fuel consumption on the dashboard display can be ‘tricked’

If a sensor that can read the temperature, gas flow, pressure or who knows what else is bad, we will wake up to see strange values on the dashboard display. Be it big or small

How to calculate the exact fuel consumption of your car

If you are among those who do not know how to accurately calculate the actual consumption of their car, we invite you to continue reading this article and follow these simple steps. It is necessary to specify that this calculation can be done only when planning a long trip, because it is necessary to travel at least several hundred kilometers or miles for an exact result.

  1. Although it is not important in the calculation of consumption, it is good to know what capacity your car tank has, to prepare the necessary money to make two full ones. This information is included in the technical manual of the car. Most cars have a fuel tank with a capacity of between 30 and 60 liters or between 8 and 15 gallons.
  2. The second step is to choose a gas station in which to fill the tank. Most pumps have a mechanism that will detect when the car tank is full. Try to fill the tank, then start the car and check if the fuel level is at the maximum.
  3. When you check that the fuel level is at maximum, write down on a paper how many liters (gallons) of fuel you have fueled and how many kilometers (miles) you have on the dashboard. These data must be as accurate as possible, for a result as close to reality as possible. If you forgot to read the number of liters (gallons) added from the pump, you can get this data from the receipt received from the gas station.
  4. Set the mileage odometer to zero when the gas tank is full. For most cars, this can be done by holding 2-3 seconds on a button on the clocks. To learn how to reset the mileage to zero, read the car manual. Do not forget this step because it is very important.
  5. Now is the time to go on a journey and drive as you usually do. Drive until the fuel level is neat the minimum limit, then stop at a gas station again.
  6. Now is the time to refill the tank, taking into account the steps described above. Note again on paper how many liters (gallons) of fuel you have fueled, how many kilometers (miles) you have traveled since you first fueled and get ready for a little math.
  7. The calculation is very simple. If at the second gas station you added “x” liters and between the first and second gas stations you traveled “y” kilometers, your consumption is (x * 100) / y. For example, if at the second gas station you supplied 35 liters and the distance between the two gas stations was 500 kilometers, your consumption is 7 liters / 100 kilometers – (35 * 100) / 500. The formula for converting this to mpg is 235.215/( L/100km) so 235.215/(7 L/100km) = 33.602 mpg.
    In other words, 35 liters of fuel mean 9.24 gallons the formula for converting liters of fuel to gallons si by dividing the number of liters by 3.785 (US gallons) and by dividing the number of liters by 4.546 (UK gallons)
    To know how many miles per gallon we have burned at a traveled distance of 500 km (310 miles) we should divide the number of miles, ie 310 miles, to the number of gallons, ie 9.24 gallons US and 7.69 UK the result will be 33.54 mpg US and 40.31 mpg in the UK

Many drivers adopt different driving modes to improve fuel consumption, such as acceleration mode, running speed, etc. Knowing the fuel consumption of your car will help you allocate a more accurate monthly budget. Keep in mind, however, that a car has a different consumption in the city and another consumption on long roads.

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