How much does my car really use, and how to measure it

The formula and the full-tank method

consumption = litres filled ÷ kilometres driven × 100 · fuel for a trip = consumption ÷ 100 × kilometres

The formula is short, but everything depends on how you arrive at the two numbers in it. The litres have to be exactly the ones you burned over those kilometres, and the kilometres exactly the ones you drove on those litres. Hence the five steps: fill to the top, zero the trip meter, drive normally, come back and fill to the top again, then divide the second fill by the distance on the meter and multiply by 100.

What matters is that the first and the second fill are the same kind of fill: the same nozzle, the same slope under the car, the same moment of stopping. A tank has no flat top, so “full” is not a point but a range of a few decilitres — and that range is the error the whole calculation lives with.

Our example: 41.3 L over 542 km gives 41.3 ÷ 542 × 100 = 7.62 L/100 km. That is not the factory figure and it is not supposed to be — it is your car, with your tyres, your load and your route.

Why a short leg lies to you

The usual mistake is not in the formula but in the length of the leg. The filling error stays roughly the same — say half a litre either way — while the distance does not, so on a short run it dominates the answer and on a long one it disappears.

In the table below the same half-litre passes through six legs. Over 120 km the answer ranges from 7.17 to 8 L/100 km — a spread of 0.83 L/100 km, enough to convince you the car drinks almost a litre more than it does. Over 542 km the same half-litre gives a spread of just 0.18.

The practical rule: measure at least one full tank, and ideally two in a row, then average them. And do not change how you drive because you are measuring — a number produced on best behaviour is a number you cannot use afterwards.

The trip computer and the pump are not measuring the same thing

A trip computer does not measure litres — it infers them from injector opening times and a calibration the manufacturer wrote in. The pump measures the volume that actually moved, and that is the only number you paid for. Which of the two reads higher cannot be guessed in advance, which is precisely why you measure. If the computer is off by roughly the same amount month after month, that is calibration rather than a fault: note the difference and work with it.

Half a litre per 100 km — whether it comes from your driving style, the season or the way you measured — sounds trivial until you multiply it by a year. Over 15,000 km, 7.62 L/100 km needs 1,143 L and 7.12 L/100 km needs 1,068 L — a difference of 75 litres. At e.g. 3.10 KM per litre that is 232.50 KM. Fuel prices move constantly: enter the current price from your own pump, this amount is only an illustration.

So the pump is the authority when you are counting money, and the computer is useful when you want to watch how your current driving moves the number.

What actually changes consumption

Consumption is not a property of the model but of what you do with it. The big movers are the things that change aerodynamic drag or the engine's operating regime: roof bars and a roof box, a loaded boot and a full car, city driving with many pull-aways, short runs on a cold engine, air conditioning in summer, and tyre pressure below the recommended value, which raises rolling resistance.

By how much, exactly — no calculator can tell you, because it depends on the car, the load and the route. But it is easy to measure: take two separate readings, one on a city tank and one on a motorway tank, and compare them.

In our example the same car drank 34.1 L over 318 km of city driving, i.e. 10.72 L/100 km, and 38.9 L over 615 km of motorway, i.e. 6.33 L/100 km. Over 15,000 km that is 1,608 L against 949.5 L — a difference of 658.5 litres, or 2,041.35 KM at e.g. 3.10 KM per litre. The numbers are illustrative; yours will differ, which is exactly why they are worth measuring.

One warning that saves time: never change two things between two measurements. If you take the roof box off and inflate the tyres at once, all you have learned is that the total is better, not which half was worth it.

From L/100 km to the cost of one specific trip

Once you have your own number, the cost of a trip is a single multiplication: consumption divided by 100, times the kilometres, times the price per litre. For 320 km one way at 7.62 L/100 km that is 24.38 L of fuel.

A return trip doubles the distance and the tolls, because the tolls are paid in both directions. For 320 km each way, tolls of e.g. 12.00 KM per direction and fuel at e.g. 3.10 KM per litre, the total is 640 km and 48.77 L: 151.18 KM of fuel plus 24.00 KM of tolls, so 175.18 KM. Split three ways, that is 58.39 KM each.

The per-person line is the only one that moves with the passenger count — the fuel and the tolls are the same however many of you are in the car, so every extra passenger shows up immediately.

Open the trip cost calculator with this article's trip →

Check your own numbers

Open the calculator with this article's example — 542 km and 41.3 L — then replace them with your own figures from the trip meter and the pump receipt. The result is your consumption in L/100 km straight away.

If there was another fill-up between the two, the measurement does not count: the calculator needs one period closed by one fill, so start again from the first full tank.

Open the calculator with this article's example →