Your trip
A tank-to-tank measurement is more reliable than the dashboard average.
How to measure it honestly
Fill the tank, reset the trip meter, drive, then fill it again. The fuel that goes in the second time is exactly what you used, and the trip meter is the distance. The one thing that must stay constant is how full "full" is — stopping at the first click every time is what keeps the number steady.
Four units, one measurement
Distance per volume (km/L, mpg) rises as the car gets more efficient; volume per distance (L/100 km) falls. They are the same fact written two ways, and the second is better at showing what a change actually saves: going from 10 to 9 L/100 km saves far more fuel than going from 5 to 4.
The dashboard reads optimistic
On-board averages tend to sit a few per cent above what a tank-to-tank measurement shows. Yours measured by hand is more trustworthy, and measuring the same way repeatedly is more useful still — a single tank tells you about that week's traffic, not about the car.
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Frequently asked questions
Q. US or imperial gallons?
Both are shown. A US gallon is 3.785 litres and an imperial one is 4.546, so the same car reads about 20% higher in imperial mpg. Quoting mpg without saying which is a common source of confusion.
Q. Why is my figure worse than the official rating?
Official ratings come from a standardised test cycle, not from your commute. Short trips, cold starts, hills, roof racks, air conditioning and hard acceleration all cost fuel that the test does not see.
Q. How far can I go on what is left?
Multiply your measured economy by the fuel remaining — but leave a margin. The last portion of a tank is the least reliably measured part, and economy drops in exactly the conditions where running low is most inconvenient.