The number on the dash is a hint, not a bill. Most trip computers average a short window: the last kilometres, sometimes the last few minutes. Traffic, a cold engine, or a hill can move that figure by a litre or more. It is useful while you drive. It is a bad number to budget with.
The figure you can actually use is consumption between two full tanks. Litres in, kilometres driven, then litres ÷ km × 100. Do that three times and you have real L/100 km — the same logic without a spreadsheet.
How to measure real consumption
- Fill until the pump clicks. Note odometer, litres, and price.
- Drive as you usually do until the tank is low. Do not “save” the test with a gentle right foot.
- Fill again, same side of the car if you can. Note the new odometer and litres.
- Consumption = litres ÷ kilometres × 100.
The first tank is often noisy (partial fill, different pump). Average tanks two and three.
A sudden jump is rarely “the computer broke”. Winter short trips, a roof box, soft tyres, a stuck thermostat, or a tank you did not fill to the click explain most spikes. A log makes the pattern obvious: same commute, more litres.
“ Real consumption is the average of full tanks, not the number that flickers on the dash after one hill.“
After a few fill-ups you also get cost per kilometre: price × litres ÷ km. That is the number that matters before a holiday or before you decide the car is “cheap to run”.
If you already use Car Reminder, log litres, km, and price at the pump. After three stops the app shows average L/100 km and cost per km — the same full-tank method, without the notebook.
