Car fuel consumption: how to calculate and compare
LLCAR Editorial Team · updated
For average consumption, divide the volume of the second full fill-up by the kilometres driven since the first full fill-up and multiply by 100. For example, 42 l over 560 km gives 7.5 l/100 km.
Consumption calculator
Formula for manual calculation: litres ÷ kilometres × 100.
How to measure consumption from fill-ups
- Fill the tank to full and record the odometer reading or reset the trip counter.
- At the next full fill-up, record the litres from the receipt and the distance since the previous full fill-up.
- For several intervals, add up the litres and kilometres, then calculate one average consumption figure.
If there were partial fill-ups between full ones, add their volume to the volume of the last full fill-up.
How to compare trips
Account separately for short trips with a cold engine, traffic, idling with the engine running, speed, load and weather. Compare intervals with similar conditions. A single trip computer reading is useful for monitoring, while a fill-up-based measurement gives an independent estimate.
If consumption has risen noticeably and remains high under comparable conditions, save your receipts, mileage and any error codes that appeared. This data helps guide the initial checks.
Why fuel consumption has increased: what to check first
If the increase is confirmed by fill-ups, start with what changed between the periods. This makes it easier to decide where to look next.
- Tires and load. Check the cold tire pressure against the placard on the vehicle. Recall whether a roof rack, trailer, or extra cargo has appeared.
- Warm-up. If the trips are similar but the engine takes longer to reach its usual temperature, record how the coolant temperature changes. With this observation, the shop can choose what to check.
- Brakes and engine operation. If the car coasts poorly, pulls to one side, or there is a burning smell near a wheel, stop driving and arrange an inspection. If the engine runs roughly or the Check Engine light is on, save the readings and trouble codes.
What you can see through OBD2
On cars where the ECU provides the 01 5E parameter, the engine's current fuel consumption in liters per hour is available. It shows how quickly fuel is being consumed at that moment, including at idle. Average consumption in liters per 100 km can still be calculated independently from fill-ups.
For comparison, record the coolant temperature, RPM, and speed if the car provides this data. For a gasoline engine, it is also useful to log the available fuel trims in a comparable mode. Trims help in searching for the cause, but the volume of fuel consumed cannot be determined from them. A missing reading does not mean zero consumption.