4 Ways To Calculate Fuel Consumption (incl. calculator)

Calculating the fuel consumption is not a difficult task at all. All that you need are the input values, the formula, and a calculator.

This article explains the different types of fuel consumption values. You will find an explanation for every value, the cases for which it is used, the calculation formula, and online calculators. Use the calculators to get a sense of the magnitude of the values.

The Absolute Fuel Consumption Value

The absolute fuel consumption value is used for calculating CO2 emissions, fuel cost and the relative fuel consumption.

It represents the amount of chemical energy which is consumed during the operation of a vehicle.

Today, my truck consumed 150l diesel.

Statement from driver Eugene at the end of his shift.

This quote is an example of declaring an absolute fuel consumption.

Because the value doesn’t tell you much, it’s not very often used in communication.

However, every fuel pump displays the fuel volume pumped. This is nothing more than absolute fuel consumption.

Display of a fuel pump showing the absolute fuel consumption
Display at a fuel pump

If you fill the tank always to the top, this value represents the absolute fuel consumption since the last fuel stop. Therefore, it becomes information about the energy fed into the engine.

The fuel pump multiplies the quantity of diesel by the price of diesel and shows how much you have to pay. No secret, this is your fuel cost. (Ok, under some circumstances, you can deduct the VAT.)

Here we have the first use case for the absolute fuel consumption. – The calculation of the fuel cost.

A second use case is the determination of CO2 emissions.

Burning 1 liter of diesel leaves 2.65 kg CO2 behind. Since this is a constant value, the government uses absolute fuel consumption for calculating the CO2 emissions of freight transport. From the absolute amount of diesel sold, they calculate the CO2 emission of the whole transport industry.

If you are interested in the CO2 emission of your vehicle, you can use the same trick. Simply multiply the diesel volume by 2.65 kg/l, and you have the result.

In my article 7 Facts About CO2 And Why It Comes Out Of The Exhaust, you find more information about CO2 and fuels used in internal combustion engines.

If we divide the diesel price by the amount of usable energy in the diesel, we can calculate an energy price. Sometimes it might be interesting to compare the prices of different types of energy.

One liter of diesel contains 34.7 MJ or 9.7 kWh chemical energy usable for combustion in an engine (calorific value).

Last but not least, absolute fuel consumption numbers are one of the inputs needed to calculate the relative fuel consumption values, therefore, we will come across this value very often.

Fuel cost and CO2 calculator

Diesel: Liter

Price: Euro / Liter.

Change the input and push the button to see your fuel and energy cost, as well as the CO2 emission.

The Relative Fuel Consumption Per Distance

Usually, you don’t just want to know the fuel consumption. You want to do something about it.

The relative fuel consumption value is used for planning, comparing, and improving purposes.

It puts the fuel volume consumed in connection to a meaningful comparison value.

In vehicles, the amount of fuel consumed is usually set in relation to the distance traveled.

By using a common distance as a reference, different values can be normalized and compared with each other. This makes fuel consumption assessable and predictable.

To plan the fuel costs, you multiply the relative fuel consumption value by the planned driving distance, and then you know the expected absolute consumption and thus also the fuel costs.

You can also compare several fuel consumption values with each other and draw conclusions. How this is done exactly, you can read in many articles in this blog.

Fuel consumption in liters per 100 kilometers

The most commonly used distance is 100 km.

The formula for calculating relative fuel consumption

The calculation is simple. Divide the fuel volumed consumed by the distance traveled and multiply it by 100. This gives you the fuel consumption in liter per 100 km.

Formula for the calculation of relative fuel consumption

Multiplying by 100 gives numbers before the decimal point. Otherwise, we would have to deal with 0 and 3 to 4 digits after the decimal point. That would be impractical.

Let’s look at the case of Eugene:

  • Yesterday, Eugene drove 500 km and used 150 liters of diesel.
  • The relative fuel consumption was 30 l/100 km.
  • Today his tour is 400 km. The truck should consume 120 liters of diesel.
  • But, at the end of the shift, it is 125 liters. The relative fuel consumption was 31.25 l/100 km.
  • So, in relative terms, the consumption was over 4% higher, although the absolute fuel consumption was lower.

On the second day, the expected value has not come into reality.

It’s always like that! Fuel consumption values always have a scatter.

This calculated relative fuel consumption is usually an average value. It includes several unknown consumption values from 100 km each. In the example, the first-day value contains 5 times 100 km and the second-day value 4 times 100 km.

I will go into detail about dealing with the scatter in the articles about the average consumption and about the correct calculation using fleet monitoring.

Relative fuel consumption in liter per 100 km calculator

Fuel Volume used:

Distance driven:

.

l

km

Relative Fuel consumption:

0 l/100 km

Online calculator – planning fuel consumption

Relative Fuel Consumption:

Planned Distance:

Cost per Liter:

.

l / 100 km

km

Euro

Absolute Fuel Consumption Forecast:

Predicted Cost:

0 l

0 Euro

Fuel consumption in miles per gallon

In the US, Great Britain, Japan, and some other parts of the world, an inverted value is used.

The distance traveled is put in correlation to the fuel consumed.

The formula for the calculation of relative fuel consumption in miles per gallon

The unit of measurement is miles per gallon.

It’s a different perspective on the same problem. This value tells the distance you can travel with one gallon of fuel.

I think that’s a very viable approach to looking at fuel economy. If you know the size of your tank volume, then you can easily find out how far you can get.

The “times 100” trick is not necessary here. With almost 4 to 5 liters of diesel, a truck can drive such a long distance that the value ends up being a number well above zero.

Why do I write 4 to 5 liters? Now, there are two different gallons. The imperial gallon with 4,54609 liters, and the US liquid gallon with 3,785411784 liters. (Source: Wikipedia)

While in the l/100 km system, the number should be as small as possible, it’s the other way around in the miles/gallon world. A large amount of miles indicates better fuel efficiency.

Once I had an experience.

I rented a car in the US, and of course, I tried to drive in an economical way

The harder I tried, the bigger the fuel economy number on the display got!

I was getting more and more frustrated.

It took me quite a long time to realize that the fuel consumption was in miles per gallon.

Relativ fuel consumption in miles per gallon calculator

Distance driven:

Fuel Volume used:

.

Miles

Gallon

Relative Fuel consumption:

0 MPG

If you want to convert from miles per gallon into liters / 100 km or vice versa, you have to differentiate whether you use the imperial gallon or the US gallon. For the US use this link, for the imperial this link.

Because of the two different miles per gallon, you can not be sure, whether any two values are comparable or not. I love metric units!

The Relative Fuel Consumption Per Time

Fuel consumption in liters per hour.

If the vehicle is operated at a standstill, the fuel consumption is related to the operating time of the engine.

The unit of measure is liters per hour.

A typical use case for this calculation method is the idle consumption of the engine.

It needs to be optimized, and therefore it needs to be measured. Shutting down the engine is the most common optimization method.

Some vehicles show this value separately on the display, others mix it up with the distance-based value, some just ignore it.

You need to make yourself familiar with the way your vehicle deals with the idle consumption value.

In case you use the distance-based calculation for your vehicle, most likely you are going to mix it up as well because you just don’t have the data to separate it. As long as you are still having an eye on your idle time, it might be acceptable.

If your vehicle is a work machine or is equipped with a work machine, you need to measure this value separately, for sure.

A typical work machine is, for example, a concrete pump. A vehicle equipped with a work machine could be a truck with a loading crane. Both types of vehicles are operated for a significant amount of time at a standstill.

Standstill consumption calculator

It is not easy to get to know the idle or standstill consumption of an engine because typically it will not be published by the OEM. You can measure it yourself, using the fleet monitoring method. Instead of driving, you idle or operate the truck under the condition for which you want to know the value.

But you can measure it yourself. Use the fleet monitoring method for this.

  • Fill the tank completely full.
  • Operate the vehicle while stationary. Recording the operating time.
  • Pour diesel into a measuring cup, use it to fill the tank up to the initial level.
  • Read off the refilled diesel volume on the measuring cup and write it down.
  • Calculate the standstill consumption.
  • Repeat the measurement on several days.
  • Calculate the average fuel consumption from the individual values.

Online calculator – relative standstill consumption

Fuel Consumption:

Operating Time:

.

Liter

Minutes

Relative Standstill Consumption:

0 Liter

Online calculator – absolute standstill consumption

Once you have the information available, you can use the calculator to calculate the amount of fuel needed.

Time:

Fuel consumption:

.

Minutes

l / h

Fuel consumed:

0 Liter

The Specific Fuel Consumption

The specific fuel consumption correlates the fuel consumed to the work output.

In contrast to the previous fuel consumption values, it is not the fuel volume that is used, but the fuel mass.

The formula for the calculation of specific fuel consumption in grams per kilowatthour

The unit of measurement is g/kWh.

The measurement is usually carried out on an engine test bench, so the calculation is based on the measured power output. Since the fuel mass is related to time, the power becomes the mechanical work in this case.

This value characterizes the efficiency of the motor.

By using the fuel mass, the influence of changes in the density of the liquid is excluded. The fuel mass represents directly the chemical energy contained in the diesel fuel. Therefore, it is energy input versus work output, and this is the definition of efficiency.

This value is the hot topic of conversation among engine developers and thermodynamicists. This is where they live out their ambition.

Usually, the specific fuel consumption is not communicated as a single value, but as a diagram with multiple values for different operating points of the engine.

This diagram is called a shell diagram. Click the link to see a picture of such a diagram.

The lines, giving it the shell look, represent lines of the same specific fuel consumption. (The pressure value on the y-axis correlates with the engine torque. Don’t be confused. It’s the small p for pressure, not the capital P for power.)

The specific fuel consumption map of an engine is a very important information for engineering smart driving and engine control strategies. Therefore, it is used very frequently by the engineers of the vehicle manufacturer. In daily operation, it is not so commonly used.

Conclusion

  • Calculating fuel consumption is easy, everybody can do it.
  • The absolute fuel consumption is a diesel volume in liters.
  • The absolute fuel consumption is used for calculating CO2 emissions, fuel cost, and for the calculation of the relative fuel consumption.
  • The relative fuel consumption in l/100 km is diesel volume divided by distance times 100.
  • It is used to plan and improve fuel consumption.
  • The stationary consumption in l/h is calculated from the diesel volume divided by the operating time.
  • It occurs when idling or when operating machines.
  • The specific fuel consumption is calculated from the diesel mass per time divided by the effective engine power.
  • The specific fuel consumption corresponds to the engine efficiency and is shown as a shell diagram.

All calculations are approximate and for demonstration only. I exclude any liability for the correctness of the result.

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