How to Calculate Fuel Consumption and Cost Per Kilometre in South Africa

Last updated: October 25, 2026

Most drivers have only a vague idea what a kilometre actually costs them. The manufacturers’ figures are measured on a rolling road in a laboratory, the fuel price changes every month, and the trip computer is optimistic. This guide shows you how to work out the real number from your own driving, in about five minutes.

Understanding What Fuel Really Costs You

Fuel is one of the biggest monthly expenses for most South African motorists, and with petrol prices that seem to climb far faster than they fall, it pays to know exactly what your driving is costing you. Yet many people have only a vague sense of how much they spend on fuel, because they have never sat down and worked out their actual consumption and cost per kilometre.

Knowing your car’s real fuel consumption is not just a curiosity. It helps you budget accurately, compare whether a more fuel-efficient vehicle is worth the upgrade, plan road trips, and spot when something might be wrong with your engine or tyres. A sudden drop in fuel efficiency is often the first sign of a mechanical problem, and catching it early can save you a fortune in repairs.

This guide will show you, step by step, how to measure your fuel consumption in litres per 100 kilometres, how to convert that into a cost per kilometre in rands, and how to use these numbers to make smarter decisions about your driving and your budget.

What Fuel Consumption Actually Means

Before we get to the calculations, it helps to understand the numbers. Fuel consumption is a measure of how much fuel your car uses to travel a given distance. In South Africa, the most common measurement is litres per 100 kilometres, written as litres/100km. The lower the number, the more fuel-efficient the vehicle.

A small, economical hatchback might use around 5 to 6 litres per 100 kilometres, meaning it travels 100 kilometres on roughly five to six litres of petrol. A large SUV or bakkie, on the other hand, might use 10 to 15 litres per 100 kilometres. The difference is enormous over a year of driving, which is why fuel efficiency is such an important consideration when choosing a vehicle.

It is important to distinguish between the official figures quoted by manufacturers and your real-world consumption. The numbers on the sticker are measured in controlled conditions and are almost always better than what you will achieve in everyday driving. Factors like traffic, driving style, load, air-conditioning use and road conditions all push real-world consumption higher. That is why measuring your own car is so valuable.

How to Measure Your Fuel Consumption Accurately

The most accurate method is a simple three-step process known as the full-tank method. It requires a little patience but gives you a reliable, real-world figure for your actual driving conditions.

Step 1: Fill Your Tank and Reset the Odometer

Start by filling your tank to the top at a petrol station and making a note of the odometer reading, or reset your trip meter to zero. It is important that the tank is genuinely full, right to the point where the pump clicks off, so you have a consistent starting point.

Step 2: Drive Normally

Now drive as you normally would until your tank is reasonably low. You do not need to run it to empty; using most of a tank over several days or a week gives a good average. The key is to drive your typical mix of city, highway and traffic so the result reflects your real usage.

Step 3: Fill Up Again and Record the Numbers

When you are ready to refill, go back to the same pump if possible and fill the tank to the same full point. Note exactly how many litres the pump says you have just put in, and note the distance you have travelled since the first fill, either from your trip meter or by subtracting the two odometer readings.

Calculating Litres Per 100 Kilometres

Now you have the two numbers you need: the litres of fuel used and the distance travelled. The formula is straightforward. Divide the litres by the distance in kilometres, then multiply by 100 to get litres per 100 kilometres.

For example, imagine you drove 480 kilometres and the pump showed you used 42 litres. You divide 42 by 480 to get 0.0875, then multiply by 100 to get 8.75 litres per 100 kilometres. That is your real-world fuel consumption.

It is a good idea to do this over several tanks and take an average, because a single trip can be skewed by unusual conditions such as a heavy load, a long highway stretch, or particularly heavy traffic. Three or four fill-ups averaged together give you a much more reliable picture of your car’s true consumption.

Converting to a Cost Per Kilometre

Knowing your consumption in litres per 100 kilometres is useful, but for budgeting what you really want is the cost per kilometre in rands. This tells you how much every kilometre you drive costs in fuel alone, which makes it easy to estimate the cost of any journey.

First, you need the current price of fuel in your area, which you can find at any petrol station or online. Let us use a simple example where petrol costs R23 per litre. Using our figure of 8.75 litres per 100 kilometres, the cost of driving 100 kilometres is 8.75 multiplied by R23, which equals R201.25. To get the cost per single kilometre, divide that by 100, giving you R2.01 per kilometre.

There is a handy shortcut. Divide the litres-per-100 figure by 100 to get litres per kilometre, then multiply by the fuel price. For our example, 8.75 divided by 100 is 0.0875, and 0.0875 multiplied by R23 is R2.01 per kilometre. This single number lets you estimate the fuel cost of any trip in seconds. A 40-kilometre daily commute would cost about R80.40 in fuel each day.

Accounting for Diesel, Petrol and Your Driving Mix

The type of fuel your vehicle uses and the way you drive play a big role in what your fuel costs you. A diesel vehicle typically returns better fuel economy on the open road than a petrol car of similar size, which is one reason diesel bakkies and SUVs are so popular in South Africa for long-distance travel. However, diesel prices can be higher per litre, and diesel vehicles often cost more to service, so the saving is not always as clear-cut as the consumption figure suggests.

Your driving mix matters just as much. A vehicle that is efficient on the highway can be surprisingly thirsty in town, because stop-start city driving forces the engine to work harder and prevents it from reaching its most efficient operating range. This is why the same car can show very different consumption figures depending on whether the driver mostly commutes through Johannesburg traffic or cruises along a coastal route.

When you are measuring your own consumption, try to capture a realistic mix of driving. If you spend most of your time in the city, measure it across a normal city week. If you are planning a long holiday road trip, it is useful to measure your highway consumption separately, because it will almost certainly be better, and having both numbers gives you a far more accurate picture of your costs in different situations.

Using These Numbers to Save Money

Once you know your cost per kilometre, you can start making decisions that genuinely reduce your fuel bill. One of the most immediate uses is comparing routes. If your commute has a highway option that is slightly longer but lets you travel at a steady speed, it may actually use less fuel than a shorter route through stop-and-go traffic, because constant braking and acceleration burn far more fuel.

The numbers also help you evaluate your driving habits. Aggressive acceleration, speeding, carrying unnecessary weight in the boot, and running the air-conditioning flat out all increase consumption. Improving these habits can cut your fuel use by ten to twenty percent without changing your car at all, which on our example vehicle means saving around 20 to 40 cents per kilometre.

Your calculations can also guide a vehicle purchase decision. Compare the real-world consumption of a car you are considering against your current one, multiply the difference by your annual kilometres, and you will see exactly how much you would save or spend over a year. This turns a vague concern about fuel economy into a concrete number you can weigh against the purchase price.

Watching for Warning Signs

Tracking your fuel consumption over time is also a powerful diagnostic tool. If your litres-per-100 figure suddenly jumps by a noticeable amount with no change in your driving habits, it is worth investigating. Common causes include under-inflated tyres, which increase rolling resistance, a dirty air filter, worn spark plugs, or a more serious engine issue.

Frequent short trips can also inflate your average, because a cold engine uses more fuel in the first few minutes of driving. If your numbers vary significantly between a week of short city trips and a week of highway driving, that is normal. But if your consumption creeps up steadily over several tanks, something may be wrong, and catching it early is far cheaper than letting a minor problem become a major repair.

The Takeaway on Calculate Fuel Consumption and Cost Per

Calculating your fuel consumption and cost per kilometre is one of the simplest and most valuable things you can do as a South African motorist. With just a full tank, a trip meter and a little arithmetic, you can know exactly what your driving costs, spot problems early, and make informed decisions about routes, habits and future vehicles.

The numbers are easy to work out once you know the method, and the insights they provide pay for the effort many times over. Take a few minutes to measure your car’s real consumption this week, and you will have a tool for smarter budgeting and smarter driving that you can use for as long as you own a vehicle.