5 Ways To Calculate Fuel Consumption (incl. calculator)

In this article, I present the different types of fuel consumption values and explain what they are used for and how they are calculated.

First, I will explain the absolute fuel consumption. It makes sense to start with it because it forms the basis for the other consumption values. Here you will also find the calculator for fuel costs and CO2 emissions.

Next, I will discuss the most commonly used value, which is the relative, distance-related fuel consumption. Here in Europe we calculate it in litres per 100 km. I will explain how it is calculated and why it is so popular. There is a calculator for calculating the litres per 100 km and a calculator for planning fuel consumption.

Then, I will look at the American and British version of relative fuel consumption. There, it is measured in miles per gallon. Of course you will find a calculator for the calculation in miles per gallon.

I have a calculator available for you to convert between different units of measurement.

Next comes the relative, time-related fuel consumption in liters per hour. There is a calculator for the relative standstill consumption and the absolute standstill consumption.

Finally, I will look at the specific fuel consumption in grams per kilowatt-hour.

In addition to calculating the consumption values, I will also cover deriving CO2 emissions from the consumption value and calculating fuel costs.

Basics

Calculating the fuel consumption is not a difficult task at all.

All that you need are the input values, the formula, and a pocket calculator, and you are ready to go.

But actually, you don’t just want to know a fuel economy value. You want more!

You want to improve your fuel economy, or maybe you just want to plan it. In general, you want to do something with the calculated value.

The calculation of the fuel economy represents a crucial intermediate step towards your ultimate goal.

To successfully complete this step, using correct input values is crucial, because if the intermediate step yields an incorrect result, your final conclusion is likely wrong as well.

The accuracy of the calculation directly depends on the data entered into the formulas and online calculators.

For this reason, it is important how the input values are determined.

Here, in this article, I explain the calculation of different types of fuel economy figures.

You’ll find an explanation of each value, the particular use cases for which it is used, and the calculation formulas.

But I also want to help you with the other steps by writing a separate article for each step that explains everything in great detail.

In most cases, the fleet monitoring method is suitable to find out the correct inüt valuesfor your real real fuel consumption. I have written an article that explains this procedure in detail. Be sure to check it out after reading this article.

Alternatively, other methods can be used to find out the input values. There exists a separate article with details about this as well.

But now, let’s take a look at the different types of fuel consumption values and their calculation.

The Absolute Fuel Consumption Value

The absolute fuel consumption shows the amount of fuel consumed by the vehicle.

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

In a diesel vehicle, it represents the amount of chemical energy which is consumed during the operation.

Calculator – Fuel cost and CO₂

Today, my truck consumed 150l diesel.

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

Because this value does not say much, it is relatively little used in communication.

However, it remains an important metric because it serves as the foundation for other consumption values.

One significant application is in the calculation of CO₂ emissions. It is also crucial for determining fuel costs and, importantly, for calculating relative fuel consumption.

How high are the fuel costs?

Each fuel pump shows the volume of fuel refilled on the display.

If you fill the tank to the top every time you stop for refueling, you are replacing the same amount of diesel that the engine consumed while driving since the last fuel stop. Therefore, the reading from the pump represents the energy fed into the engine.

This reading on the fuel pump is then directly the absolute fuel consumption value.

The fuel pump multiplies the diesel volume by the diesel price and shows how much you have to pay.

No secret, this is your fuel cost.

Well, in commercial use you can usually deduct taxes. You can use my calculator to make this adjustment.

This was the first use case for the absolute fuel consumption value. – The calculation of the fuel cost.

How high are the CO₂ emissions?

A second use case is the determination of CO₂ emissions.

Burning 1 liter of diesel leaves 2.63 kg CO₂ behind.

If the CO₂ effect of other substances produced during combustion is also taken into account, the value rises to 2.66 kg CO₂e.

This value is called CO₂-equivalent and is indicated by the small “e” after the CO₂.

Since this is a constant value, the government uses absolute fuel consumption for calculating the CO₂ emissions of freight transport.

One can take the amount of diesel sold, multiply it by the CO₂ value, and get the CO₂ emissions of the entire transportation industry.

This method allows the Federal Statistical Office to determine the proportion of freight transport’s contribution to the CO₂ issue without requiring knowledge of the consumption of each individual vehicle.

My calculator uses the same trick to calculate the CO₂ emissions of your vehicle. It multiplies the diesel volume by 2.63 respectively 2.66 kg/l.

In the article: 15 answers about CO₂ incl. why it comes out of the truck exhaust, you will find more information about CO₂ and CO₂ emissions when using different fuels in combustion engines. There you find also the sources from which the values come and other values.

What is the energy price of diesel?

If you divide the price of diesel by the usable energy contained in one liter of diesel, you get the energy price.

This is not an absolute value, but a relative one, but I find it helpful when I want to compare the price of different forms of energy.

One liter of diesel contains 35.5* MJ or 9.8** kWh of chemical energy usable for combustion in an engine (calorific value).

*source: https://www.chemie.de/news/131996/der-energiegehalt-im-kraftstoff.html **source: https://www.energie-gedanken.ch/umrechnungsfaktoren/

You can easily compare the price ratio of 1 kWh of diesel energy to 1 kWh of electric energy.

How many kWh of energy did you use?

I suppose it’s also interesting to you how much kWh of energy was consumed.

That’s why I let the calculator work it out and write it in parentheses in the answer sentence.

Use the absolute consumption to calculate the relative consumption.

Finally, perhaps the most important application for absolute fuel consumption:

It is an important input parameter for the calculation of relative fuel consumption.

For this reason, we will come across this value very often.

And that’s a perfect segue to the next fuel consumption type.

Calculate The Relative, Distance-Related Fuel Consumption In l/100km.

Calculator – Relative fuel consumption in liters per 100 km

Calculator – planning fuel consumption

The formula for the calculation of relative fuel consumption in l/100 km.

Divide the fuel volume consumed by the distance traveled and multiply it by 100.

The result is the fuel consumption in liter per 100 km driving distance.

Multiplying by 100 gives numbers before the decimal point.

Without this trick, we would have zero, decimal point, and then decimal places. That would be inconvenient.

Well, another alternative would be to express fuel consumption in milliliters or shot glasses per kilometer. However, l/100 km is definitely more practical.

The relative, distance-related fuel consumption in l/100km

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.

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.

This calculated relative fuel consumption is usually an average value. It includes several unknown consumption values from 100 km each.

You have not only driven 100 km since the last refueling but there were several 100 km intervals between the last and the current refueling.

I say this because there is another average consumption, namely the average consumption, which can be calculated from several of the relative consumption values.

When people talk about average consumption, they are often referring to the value that we calculate here. However, we usually need the other version of average consumption, with the help of which we can achieve the required accuracy.

Relative fuel consumption, as we calculate it here, is generally a good starting point. However, we need to check whether it is accurate enough to use it for comparisons and conclusions.

This will not usually be the case.

The problem is the scatter, which possibly distorts the value. It depends very much on how the input values were determined.

As already written at the very beginning, the calculation of the fuel consumption value is only an intermediate step on the way to the correct result. Therefore, be sure to read the fleet monitoring article!

There you will learn how the input values for the calculation are determined.

In the article: “How to assess the distribution of fuel consumption values?”, you can read everything about scattering. If you want to delve a little deeper into the theory, you can do so in the article on the normal distribution.

The article: “How to calculate the average consumption correctly.” deals with the calculation of the final, usable average consumption.

This is the other, really relevant average consumption value that we need.

Why relative consumption is a good comparison value.

That sounds complicated, then why is the relative consumption value actually a good consumption value?

Let’s take another look at the case from above:

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

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

Plan fuel consumption.

You will have noticed that I have just planned the fuel consumption for the second day.

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.

The “Calculator – planning fuel consumption” is doing just that.

How much CO₂ does a truck emit per 100 km?

Since this value is so easy to calculate, I have included it in the consumption calculator above.

This allows you to see the CO₂ emissions for your specific consumption directly in the result.

If you want to enter a relative consumption, then enter the value in l/100 km in the field for the consumed fuel volume and set the distance to 100 km.

Calculate The Relative, Distance-Related Fuel Consumption In Miles per Gallone.

Calculator – Relative fuel consumption in miles per gallon

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

The distance driven is divided by the volume of fuel consumed.

In the formula for fuel consumption in miles per gallon, the distance value is located above the fraction line and the volume of fuel consumed is below the fraction line.

The relative, distance-related fuel consumption in miles per gallon

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

It’s a different perspective on the same problem.

The constant under the fraction line is therefore not 100 km but 1 gallon of diesel.

I believe this is a highly practical method for evaluating fuel efficiency.

Knowing the size of your fuel tank allows you to determine your potential travel distance easily using this approach.

Multiply the removable tank volume by the relative consumption and you have the distance to the next fill-up.

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.

What to look out for when comparing MPG values!

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)

So, since there are two different values with the same unit of measurement “miles per gallon”, you can’t be sure if the values are comparable.

I love metric units!

By the way, in Japan, fuel consumption is given in kilometers per liter.

Calculator – Conversion of relative fuel consumption

If you want to convert from miles per gallon to liters per 100 km or vice versa, you have to distinguish between the imperial gallon and the US liquid gallon.

In the following calculator you can enter your fuel consumption value, select the unit of measurement and the corresponding values will be displayed in the other units of measurement.

Calculate The Relative, Time-Related Fuel Consumption.

Calculator – Relative standstill consumption

Calculator – Absolute standstill consumption

The formula for the calculation of relative, time-related fuel consumption in liters per hour.

The relative, time-related 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.

When you calculate your distance-related fuel consumption from the volume of diesel you have filled up, you are very likely to mix in the stationary consumption.

Usually, it’s not possible to distinguish when refueling, how much of the consumed fuel volume was used while driving, and how much while idling.

If it’s only about idling consumption, then that might be fine.

If the vehicle is a concrete pump or has a loading crane that is used for a lot of work, then things are different.

Trucks are often used as work machines if they are equipped with appropriate superstructures. In this cases it makes sense to know the stationary consumption separately.

For some vehicles, the stationary consumption can be read separately in the display.

But, let’s take a quick look at the idling consumption.

The idle consumption

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

Idle consumption is critical in two respects.

1. If the engine runs at idle speed without having to deliver work, the efficiency is zero. This means that energy is uselessly converted into heat.
2. In EcoRoll mode, the engine runs at idle to drive the auxiliary consumers. The idle consumption reduces the consumption savings here.

OK, the second one is not really a stationary consumption, because the vehicle is rolling.

How much is the idle consumption of a truck?

The idling consumption depends on various factors. The power consumption of the auxiliary consumers driven by the engine, the friction in the engine and the operating temperature are key factors.

The simplest, most reasonable and most practical way for everyone to reduce idle consumption is to switch off the engine when the vehicle is stationary.

The question of why no truck manufacturer turns off the engine during Eco-Roll will be thoroughly addressed in an article. If you’re interested, you can read it for more information.

The standstill consumption

If your vehicle is a working machine, then you definitely need to calculate the stationary consumption separately.

It definitely makes sense to know and optimize this consumption.

The calculation is simple, but how do we get the input values?

How to determine the stand consumption.

It is often not so easy to get information from a manufacturer about the stand consumption.

But you can measure it yourself.

The procedure is similar to the consumption measurement drive method.

You have to be very pedantic here to get a correct value.

If you have any questions about this, write me and we’ll look at it together.

The Specific Fuel Consumption

The formula for the calculation of the specific fuel consumption.

Here, the mass of fuel consumed is divided by the effective engine power.

The unit of measurement is g/kWh.

Specific consumption describes the fuel efficiency of internal combustion engines.

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

This value basically describes the efficiency of an engine.

Above the fraction line is the energy that is fed into the engine. (Chemical energy in the form of fuel)

To ensure that the energy is really used as the input value, the fuel mass is used here. It correlates directly with the chemical energy.

The amount of fuel volume consumed not only depends on the energy content but also on density, which is, in turn, influenced by temperature. This complexity is avoided by using the fuel mass.

Under the fraction bar is the submitted work.

So the energy in through work out = efficiency.

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

How is specific fuel consumption determined?

I don’t have a calculator here because this value is measured on an engine dyno and the engine dyno software outputs this value directly.

You see in the formula that on the test bench the power output of the engine is measured.

In conjunction with the fuel mass per unit of time, this then results in the desired efficiency value.

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.

The lines that give the diagram its scalloped appearance are lines with the same specific fuel consumption.

The pressure value on the y-axis corresponds to the engine torque. Don’t get confused, it’s the small p for pressure, not the big P which usually stands for power.

On the x-axis is the speed. This means that every point in the diagram is a power point.

If the data is saved in the form of a table, then you have the fuel consumption map of an engine.

The specific fuel consumption map of an engine is very important information for engineering smart driving and engine control strategies.

Here you can see at which operating points the engine efficiency is best.

This map is also imported into the calculation tool for calculating the VECTO consumption value

The fuel consumption map is used very intensively by vehicle manufacturers’ development engineers.

It is rarely used in normal driving.

Conclusion

If you have any questions or need help, write me and I will try to help you.

Please be sure to write me if you find any errors that I need to correct.

I exclude any liability for the correctness of the result.