Fuel costs on the rise? Calculating your car's fuel efficiency can help you save money at the pump. This guide walks you through three methods:
1.) Car fuel consumption using your car's trip computer (if equipped)
2.) Car fuel consumption manual calculation method
3.) The drive-till-it's-empty method.
Take the Quiz: What's Your Fuel Savvy Score?
Answers
| Correct Answer |
| a) Litres per 100km (L/100km) |
| False. Manufacturer figures are often optimistic. |
| b) 14.3 km |
Don't worry if you didn't get a perfect score! This guide will teach you everything you need to know. We'll walk you through three easy methods to calculate your car's fuel consumption, giving you the power to take control of your budget.
Fuel consumption formula
If litres per 100 km figures don’t make sense to you, you can use an online converter to find out how many kilometres you’ll get on one litre of petrol, and vice versa. Or, you can just do a simple calculation as per the table below.
To convert Formula to use
km per litre to litres per 100 km | divide 100 by km per litre |
litres per 100km to km per litre | divide 100 by litres per 100 km |
Working out your car's fuel consumption using a car trip calculator
It may sound like a difficult and complex task, but we guarantee you, it isn't. Even this writer, with a very limited grasp of mathematics, manages to do it!
Today, most cars are issued with a nifty little piece of equipment called a car trip computer.
Most modern trip computers will record, calculate, and display the distance travelled, the average speed, average fuel consumption, and instant fuel consumption.
In most cases, it will tell you your range, i.e., how many more kilometres you can travel before you need to fill up. However, if your car is slightly older and without a trip computer, you will have to rely on simple mathematics to get a more realistic idea, which we will cover in this article.
Related: These are some of SA's most fuel-efficient cars
How many litres of fuel per kilometre, or litres per 100 km, does my car use?
Before we start, let's find the answer to this question in order to start our calculations.
As a car journalist, I mostly work with litres per 100 km, but many people understand the km-per-litre system better. In this image above, you'll see a figure of 13.7 km/l. That means this car uses 13.7 kilometres per one litre of fuel.
Related: 5 Used Diesel SUVs with Excellent Fuel Economy
1. How to work out your car's fuel consumption with the help of the trip computer
Let’s say, for example, that your trip computer (or the *manufacturer's website) says your specific car uses 8.0 litres of fuel per 100km. The actual formula to get km per litre is 100 divided by 8 = 12.5, meaning that your car will give you 12.5 km per one litre of fuel.
(Still unsure? I also used two websites called convertworld, and convert-me, and entered the relevant details.)
Review: Mahindra XUV300 1.5TD W8 - Your Secret Weapon Against Fuel-price Hikes
2. How to work out your car's fuel consumption without a trip computer
The first step in working out consumption is to know that the equation is KILOMETRES DRIVEN, DIVIDED BY THE AMOUNT OF FUEL USED.
If you know the exact distance you covered and how many litres it takes to fill up your tank (i.e., your fuel tank size, also on the *manufacturer’s website, usually), you can easily just divide the kilometres driven by the fuel used to get your km-per-litre result and just convert that to litres per 100 km.
Here is an example: According to Suzuki, the fuel in the previous-generation Baleno’s 37-litre tank will keep you on the road for 726 km before you’ll need to fill up.
726 km divided by 37 = 19.62 km per litre. This translates to usage of 5.097 litres per 100 km.
Having driven this Baleno, I know the real (combined/average) figure is actually 5.8 litres per 100 km (100 divided by 5.8), which translates to 17.24 litres per km.
To know how far I’d be able to travel on a single tank, I will then multiply 17.24 by 37 (the number of litres the tank holds). So, in real-life conditions, based on average consumption of 5.8 litres per 100 km, I will be able to cover a distance of 637.88 km.
Related: Should you buy a car with high mileage?
3. The drive-till-it's-empty method of calculating fuel consumption
Another no-brainer method would simply be to see how far you can drive on a single tank. Here's what to do:
- When your tank is very close to empty, reset your odometer to zero.
- Now fill your car up completely. (If you don’t have a trip odometer, just make a note of the total km travelled just before you put in fuel.)
- Now, drive your car until it’s as close to empty as possible, making another note of the total km before filling up again and recording the number of litres used to fill up the tank.
- Then divide the number of kilometres by the litres used to get kilometres per litre, and convert it to litres per 100 km, which is 5.8 km/L (if you divide by 37).
Read our review of the fuel-efficient Renault Duster 1.5 dCi Intens EDC 4x2
Estimating the fuel cost for a trip
I love road trips, and I am always looking to save money on fuel (so I can spend it on food!), so I usually work out my petrol budget to the last kilometre.
How to calculate potential fuel usage:
- To estimate the fuel cost for a trip, you need the trip distance, the cost of fuel per litre, and the vehicle’s average fuel consumption.
- Divide the total distance (km) by 100. Now multiply the answer by the average fuel consumption, and then multiply this number by the price of fuel (per litre).
Here's an example:
Let’s say I want to travel from Cape Town to East London, which is a distance of 1040 km. I will be using the above-mentioned Suzuki Baleno because it’s light on fuel. To make this calculation, I will use the average fuel consumption of 5.8 litres per 100 km, but then I also have to take into consideration that I probably won’t be driving 90 km per hour in perfectly flat, windless conditions. So I will base my calculations on 7.0 litres per 100 km. I’d rather be a little conservative so I’m not out of pocket should I encounter typhoons blowing against the front of my windscreen (hopefully not)!
Ok, let's calculate our probable fuel consumption!
- My Baleno will use 7.0 L/100 km. So the first step is 1040 km (the distance) divided by 100, which equals 10.4. Now I will multiply this by 7, which equals 72.8 litres.
- We will now multiply this figure of 72.80 by the current fuel price. I used the current price (93) at the coast, which is R 25.17 (correct on the 3rd of January 2024), which amounts to R 1,832,37.
- Should you plan on coming back, double that figure, but don’t forget about the kilometres you’ll be covering while you’re there!
And remember, almost everything is always more expensive than you think. It’s a sign of the times. Happy travels!
How much does it cost to fill SA's best-selling vehicles? Find out here.
*A note about the manufacturers' quoted fuel consumption figures
If you go according to the manufacturer’s consumption figures, remember that they are (mostly) a little optimistic and their economy runs are often done in perfect conditions, yielding a much lower result (remember Dieselgate?).
For example, when a manufacturer claims a certain car uses 5.0 litres per 100 km, you can almost add 2 litres to that figure for a more realistic idea of fuel economy.
In our experience, however, Suzuki's figures are usually spot-on, which is why we used the Baleno for our calculations.
Want to sell your current gas guzzler and buy a frugal cheapie instead?
If you are looking to sell your current car quickly and easily, why not try our Instant Offer here? You can search for new or used fuel-efficient models on AutoTrader here.
