how to calculate fuel consumption in

Fuel consumption in cars is one of the most important factors to know while owning and driving a car.

This knowledge is more or less necessary for all drivers and car owners as it can make the difference for important situations like: having a healthy home or company budget , should you replace your present car or not, condition of the engine etc.

These reasons should be good enough for you to learn this, especially since it’s not that hard to do.

As you probably already know, fuel consumption is measured in liters per traveled 100 km or in miles per gallon. Whatever metric system you use in your country, these tips should be applicable.

The most important thing in this article is learning and understanding the PRINCIPLE of calculating fuel consumption.


What will you need to calculate fuel consumption in cars?

All you’ll need is :

  • Full tank of fuel-one or more.

full gas tank-how to calculate fuel consumption in

  • Persistence and time

It will take some time to get accurate data. Depending on how much you use your car and how much fuel you spend, it may take a few weeks to a few months of monitoring. Accurate fuel consumption is measured both through city and long distance driving.

So, no persistence, no accurate fuel consumption.

  • Taking down notes

It would be best to take a separate note-book or a piece of paper for accurately writing down the collected data. Of course, there is the smart phone or tablet. Your choice.

  • Simple mathematics

Once you’ve collected enough data, use a simple mathematical formula which will give you accurate fuel consumption of your car.

What are the two best methods for calculating?

Two methods that I can recommend (from my own experience):

  • Monitoring the amount of fuel your car has consumed and the traveled distance.


  • Monitoring the on-board computer data for a prolonged period of time.

onboard computer-how to calculate fuel consumption in

 How to calculate fuel consumption in cars(classic way)?

Two ways of doing this:

  • Shorter method:
  1. Drive until the low fuel warning light comes on.
  2. Fill up the gas tank (don’t overdo it,first pop of the fuel pistol will be enough)
  3. Drive 100 kilometers.
  4. Refill the fuel tank again (till the first pop of the fuel pistol)

The amount of fuel filled up is the fuel consumption for 100 traveled kilometers.

So, if you’ve poured 7 liters of fuel after 100 kilometers that’s your average fuel consumption per 100 kilometres.

  • Longer method
    1. Drive until the low fuel warning light comes on.
    2. Fill up the gas tank (don’t overdo it,first pop of the fuel pistol will be enough).
    3. Write down how many liters of fuel you’ve poured in the tank.
    4. Reset the trip distance on the odometer.
    5. Drive until you spend the fuel and the low fuel warning light comes up again.
    6. Write down the distance.

 Your log should look something like this (I’ve written the data for example):


Month Traveled distance (kilometers) Liters
January 1000 70
February 1200 80
March 900 60
April 1500 90
Total : 4600 300


Add up the traveled distances and the liters (total).

In this example the car has used 300 liters of fuel for 4600 traveled kilometers in four months.

In order to find the average fuel consumption for this period, divide liters by the distance:

300 liters/4600 kilometers=0.0652 liters (this is the consumption per 1 kilometer)

Then multiply this figure by 100:

0.0652 x 100= 6.52 liters.

This is the average consumption of the car in this example.

 How to calculate fuel consumption using the onboard computer?

This is the easier method. Again, you’ll need persistence and writing down the collected data.

On-board computers have two “layers” of recording fuel consumption and travel distance:

  • Short term time and distance
  • Long term time and distance

Long-term data is what you’ll need to be looking at. If you can’t reach these settings on your car model, it would be best to look up the user’s manual or on the internet.

Once you’ve sorted this out, this is what you’ll do:

  1. Drive until the low fuel warning light comes on.
  2. Fill up the gas tank (don’t overdo it, first pop of the fuel pistol will be enough)
  3. Reset the on-board computer-long-term settings for both fuel consumption and trip distance. You can also reset the trip distance on the odometer, if you want double tracking.
  4. Drive until you spend the fuel and the low fuel warning light comes up again.
  5. Note the distance and average consumption. Write it down. More notes per one spent fuel tank means more accuracy.

Again you should write down the data:


Month Traveled distance (kilometers) Average consumption (liters/100 km)
January 1000 7.5
February 1200 7.9
March 900 8
April 1500 7.1
Total : 4600 30.5


As you see, the figures for the kilometers and average consumption are added (total).

Take the added figure for average consumption and divide it by the number of months:

30.5/4 months=7.625 liters.

The car, in this case, has an average fuel consumption of 7.625 liters for this period of time.

Additionally, if you want to find out how much liters of fuel the car has spent in this example:

4600 kilometers/100 (this is liters per 100 kilometers)=46

46 x 7.625 litres (average consumption)=350.75 liters of fuel.

Additional fuel consumption tips

  • If you want, use both methods and cross-reference the fuel consumption data

This is just a suggestion and isn’t necessary.

But If you want perfection and the ultimate fuel consumption picture, use both methods.

If everything is OK, they should be similar but not exactly the same (at least that’s what I’ve experienced).

A small percentage of deviation from one another is OK (about 0.5 liters per 100 kilometers).

  • You’ll always know how much fuel money you’ve spent

Once you know the amount of fuel, you’ll know how much money is spent.

Liters  x  price of fuel = amount of money spent

So, as an example, if you’ve used 70 liters of fuel and let’s say the fuel price is 2.20 Euros:

70 x 2.22 = 155.40 Euros.


One more useful tip: monitoring the fuel consumption of your car can help you to increase your car’s fuel efficiency.

Why should you calculate your car’s fuel consumption?

  • Planning your home or company budget

Lets face it, car fuel consumption in your household or company is a major factor. In lots of budgets it’s the difference between going under or being profitable or not. When you know the average fuel consumption of your car(s), you know how much money you spend or will spend in the future. Knowing all the financial facts means making and planning a quality budget.

  • Planning long distance travel with your car

In today’s abundance of fuel stations (almost everywhere in the world), planning your trip regarding fuel consumption seems unnecessary. When the fuel gauge needle goes down, simply stop and refill the fuel tank. But eventually, you’re route will lead you to more secluded places where fuel stations are more apart (in some places even a couple of hundred kilometers). In this case knowing the average fuel consumption of your car can be a big help (difference between left stranded and having a comfortable trip). If you’re fuel tank doesn’t have the capacity for enough fuel, you’ll know that and bring an additional container with fuel for that purpose.

This tip goes double if you use “rare” fuels such as LPG. These refill stations are still pretty scarce in most parts of the world. Planning here is a “must”, not a “maybe”.

  • Fuel consumption: useful “health “indicator of your car

Over time, you’ll notice that the fuel consumption, if everything is OK with the car, doesn’t vary much.

Having this in mind, any big variation is a sure sign that something is wrong (for instance if the average fuel consumption of the car is 7 liters per 100 kilometers and it goes to 9 liters, it’s time to raise the red alert).

Of course, you’ll also notice by the fuel gauge and the amount of fuel money spent, but it can be a gradual process which builds up over time causing you unnecessary expenses.

This can be caused by things like bad combustion (spark plugs, air-fuel mixture etc), small leaks (you’ll notice the big ones) or simply that the overall engine condition is getting weaker.

If this happens make an inspection on your own or visit a mechanic as soon as possible.

  • Important factor for deciding on whether to keep the car or not

Most drivers and car owners pay little attention to the fuel consumption factor.

But high fuel consumption (more than average for you car) means lost money.

If you’re like most drivers (among which I count myself), the car is a mean of transportation that has to be economical in order to have its place in the household.

Otherwise, it’s just money out the window.

So, if the car spends more fuel than average, you’ll know that on time in order to take preventive action (repair, sell the car or other).