How to work out your car’s real fuel consumption

The consumption formula

Fuel consumption is a division. You need two numbers — the litres you used and the kilometres you drove — and the sum is this:

consumption (l/100 km) = litres ÷ kilometres × 100

If you used 35 litres to drive 500 km, consumption is 35 ÷ 500 × 100 = 7 l/100 km. You can run this directly in the fuel consumption calculator.

The two units: l/100 km and kilometres per litre

The same measurement is expressed in two ways, and both are in use. A car’s spec sheet speaks in litres per 100 km, but the question is almost always asked the other way round: how far does it go on a litre? That is the inverted division:

kilometres per litre = kilometres ÷ litres

Over the same 500 km on 35 litres, that is 500 ÷ 35 = 14.29 km/l. The two units are reciprocal, and the bridge between them is simple:

km/l = 100 ÷ consumption in l/100 km

It is worth fixing in mind that they run in opposite directions: in l/100 km, lower is better; in km/l, higher is better. A car at 5 l/100 km does 20 km on a litre; at 10 l/100 km it does only 10 km.

How to measure properly: full tank to full tank

The arithmetic is trivial. What ruins a measurement is collecting the numbers, because the litres and the kilometres have to cover the same stretch of driving. The reliable method is this:

1. Fill the tank to the brim, until the pump cuts out.
2. Reset the trip meter to zero.
3. Drive normally until you need fuel again.
4. Brim the tank again and note the litres of that fill-up and the kilometres on the trip meter.

The litres of the second fill-up are exactly what you used over the kilometres on the trip meter — that is what makes the measurement a measurement. If either fill-up leaves the tank short, the litres stop matching the kilometres and the result means nothing.

One caveat about the sample: a single tank reflects the driving you did on that tank. A week of motorway and a week of city traffic give quite different figures in the same car. To learn the car’s consumption, rather than that week’s consumption, repeat the measurement over several tanks.

What it costs: per kilometre and per 100 km

With the price per litre, consumption turns into money. The cost of every 100 km is the consumption times the price:

cost per 100 km = consumption (l/100 km) × price per litre

At 7 l/100 km and €1.80 a litre, every 100 km costs €12.60 — about €0.126 per kilometre. You can also read the sum backwards and ask how far a euro goes: if 500 km cost €63, each euro buys nearly 8 km, so €20 takes you about 159 km.

Because fuel prices move almost daily, use the current price of the fuel your car takes; in Portugal the daily average price is published on the DGEG fuel-price portal¹. And if what you want is to estimate the spend on a specific trip using this consumption, that is what the fuel cost calculator is for.

Real consumption against the spec sheet (WLTP)

The spec sheet gives the combined WLTP consumption², measured on a standardised test cycle. Real consumption depends on things the test does not reproduce: speed, braking, traffic, gradient, the load you carry, air conditioning, tyres and even temperature. So it is normal for the real figure to sit above the homologated one.

The spec-sheet value does not exist to predict your consumption: it exists to compare cars with each other on equal terms. The useful comparison is a different one — the gap between your real consumption and the homologated figure, in litres and as a percentage. If you measure 5.57 l/100 km in a car homologated at 4.9, you are 0.67 l/100 km above it, roughly 13.6% more. It is that gap, tracked over time, that shows whether something has changed in the car or in how you drive.

Range on a full tank

Once you know the real consumption and the tank capacity, range is a multiplication:

range = kilometres per litre × litres in the tank

At 14.29 km/l, a 50-litre tank gives about 714 km. Two honest caveats: the sum assumes a completely full tank and the consumption you have just measured, and the reserve is not meant to be run to the last drop. Always leave a margin, particularly because consumption rises on the motorway — exactly the kind of driving where you would use the whole range.

What about an electric car?

It does not apply. An electric car is measured in kilowatt-hours per 100 km and paid for at an electricity tariff, not by the litre. The sum has the same shape, but the units and the price are different: see the charging cost calculator.