How to work out your car’s real fuel consumption: full to full
The trip computer gives you an estimate. Your real consumption comes from two things you already have: the odometer and the fuel receipt. You just need to use them in the right order.
The method in three steps
- Fill the tank and write down the odometer reading. This is the starting point — nothing is calculated from this fill yet.
- Drive as usual until your next fill-up.
- Fill the tank again and write down the odometer and the litres. What went into the tank is exactly what the car used since the last full tank.
Example: 610 km since the last full tank, and 42.5 litres to fill it up. Consumption is 42.5 ÷ 610 × 100 = 6.97 l/100 km.
What “full” means
Up to the first click of the nozzle — no topping up to the brim. The point is to fill the tank the same way every time. If you can, use the same station, ideally the same pump.
Partial fills: don’t calculate, carry the litres forward
When you add only some fuel, the tank is full neither before nor after, so that fill alone cannot tell you anything. The result can be absurd: 5 litres added after 200 km would suggest 2.5 l/100 km — and the next full tank would then show a car that suddenly drinks for two.
The fix: add the litres from a partial fill to the next full tank, and measure the distance from the last full tank.
| Fill-up | Odometer | Litres | Consumption |
|---|---|---|---|
| Full | 10,000 km | — | starting point |
| Partial | 10,300 km | 20.0 | not calculated — litres carried forward |
| Full | 10,650 km | 25.0 | (20 + 25) ÷ 650 × 100 = 6.92 |
Average consumption: total over total, not an average of results
This is the mistake most spreadsheets make. Take two legs: 100 km on 10 litres (10 l/100 km) and 600 km on 42 litres (7 l/100 km). The average of those two results is 8.5 l/100 km — but the car really used 52 litres over 700 km, which is 7.43 l/100 km.
Averaging results gives a short leg the same weight as a long one, and short legs are the least accurate: the nozzle clicks off a litre early or late and the result jumps. So always work out the average as all the litres divided by all the kilometres.
Why the tank size matters
Not for the formula — the capacity says how much the tank holds, not how much was in it. It is useful for two other things:
- Range on a full tank: capacity ÷ consumption × 100. A 50-litre tank at 7 l/100 km gives about 714 km.
- Catching typos: if you type 599 litres instead of 59.9 and the tank holds 55, the mistake is obvious at once.
The most common mistakes
- A typo in the odometer reading spoils two results — that reading ends one leg and starts the next.
- Calculating consumption from the first fill. The first full tank is only the starting point.
- Treating a top-up as a full tank. One such fill inflates one result and deflates the next.
Keeping track without a spreadsheet
In Portfolio Tracker you log each fill-up on the Vehicle tab: odometer reading, litres and cost. A FULL / PARTIAL switch tells the app whether to carry the litres forward. The average is worked out as total over total, and once you enter the tank size you see your range and get a warning for a fill that would not fit in the tank. The cost of fuel goes into the month’s expenses automatically, alongside your cost per kilometre. Prefer miles? The tracker also works in miles, US gallons and mpg.
Educational material. All figures in the examples are for illustration.