How to calculate your car's fuel consumption

The trip computer shows one figure, the brochure claims another and the fuel receipt tells a third story. The most reliable way to know what your car really uses takes two readings: the full-tank method. All you need is a tank filled until the nozzle clicks off and your odometer.

The formula

Consumption (L/100 km) = litres added ÷ kilometres driven × 100

The litres are those you put in between two full tanks, the second one included. The kilometres are the difference between the two odometer readings.

The full-tank method, step by step

1. Fill up until the nozzle clicks off and note the odometer, for example 42,310 km. This fill-up is your starting point: its litres don't count.
2. Drive as usual.
3. At the next full tank, note the litres added and the odometer, for example 41.6 L at 42,960 km.
4. Work it out: 41.6 ÷ 650 × 100 = 6.4 L/100 km.

A partial fill-up in between changes nothing as long as you add its litres. With 20 L halfway and then 21.6 L at the full tank, the total is still 41.6 L and the result 6.4 L/100 km. What matters is a full tank at the start and a full tank at the end.

Why you should average over several tanks

The nozzle doesn't always click off at the same level: depending on the pump, the slope of the forecourt or diesel foam, there can be a litre or two of space left. In the example, 2 litres less at the second fill-up gives 39.6 ÷ 650 × 100 ≈ 6.1 L/100 km instead of 6.4, a 5% error on a single tank. Over five or six tanks, these errors cancel out.

For the average, divide the total litres by the total kilometres rather than averaging each tank's figure. A 300 km stretch at 7.0 L/100 km and a 700 km stretch at 5.0 L/100 km give (21 + 35) ÷ 1,000 × 100 = 5.6 L/100 km, not 6.0: the longer stretch weighs more, as it does in real life.

What each kilometre costs in fuel

Same logic with money: fuel cost per km = amount paid ÷ kilometres driven. With 41.6 L at €1.75 a litre, the fill-up costs €72.80, so 72.80 ÷ 650 ≈ €0.11 per kilometre.

Fuel is only part of the cost. For insurance, maintenance and depreciation, read the real cost of a car per kilometre or get an estimate with the car cost calculator.

Converting to mpg or km/L

The same mpg figure therefore means different things in the UK and in the US. UK pumps sell fuel by the litre, so if you measure in miles, convert the litres to UK gallons (divide by 4.546) before dividing the miles by them.

Why you use more than the official figure

Official consumption is measured in a laboratory on the standard WLTP cycle. The European Commission compares it with data from the fuel consumption meters fitted to new cars. According to its July 2024 publication, for cars first registered in 2022, real-world consumption exceeded the WLTP value by:

• 21.1% on average for petrol cars, or 1.3 L/100 km more;
• 17.1% for diesel cars, or 1.0 L/100 km more.

For plug-in hybrids the gap is much wider: real-world CO2 emissions of models registered in 2021 were on average 3.5 times higher than their type-approval values.

So if your car is rated at 5.3 L/100 km (WLTP), a real figure around 6.4 L/100 km is nothing unusual. Short trips with a cold engine, motorway speeds, air conditioning and under-inflated tyres widen the gap: see our 10 tips to save fuel.

Is the trip computer good enough?

It shows a good trend, and the Commission measures the gap above from those on-board meters. The full-tank method is still worth doing: it counts the litres you actually paid for and doesn't depend on a reset you forgot. Comparing the two now and then tells you how far your trip computer is off on your car.

Tracking your consumption in Staticar

The Staticar app does this calculation for you, fill-up after fill-up:

• Log each fill-up with its litres ("Price/L" or "Total" mode). A fill-up entered with only the receipt total can't give the consumption for that stretch.
• Enter the odometer: the field recalls your last reading and suggests an estimate for today.
• Keep "Full tank" ticked when the tank is full. Untick it for a partial fill-up: its litres count towards the next full tank.
• Forgot a fill-up? Tick "I forgot to record the previous refuel": the calculation restarts from this fill-up instead of underestimating consumption.

Each full tank shows its consumption from the second full tank with an odometer reading, and the Statistics tab shows your 12-month average and fuel cost per kilometre. For a vehicle set to miles, Staticar counts in miles and US gallons and shows US mpg. The app tracks liquid fuels in litres: charging an electric car in kWh is not supported.

Sources

• European Commission, real-world fuel consumption data of cars and vans collected in 2022, published 26 July 2024.