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Two measurements make a fuel economy figure: how far you went, and how much fuel it took to fill the tank again. Everything below is worked out from those two, and the answer is given in all four units at once so that no gallon goes unnamed.
Off the trip meter, reset at the previous fill.
What the pump dispensed, not the rated capacity of the tank.
A US gallon is 3.785 litres. An imperial gallon is 4.546.
27.37 mpg (US)
The same tank, said four ways, below.
The two gallons are not the same size, and the gap is about 20%. The imperial figure above will always look better than the US one for the very same tank of fuel, so a figure quoted without its gallon tells you nothing you can compare.
Litres per 100 km runs the other way: it counts fuel over a fixed distance, so a smaller number is a better car. It is also the only one of the four you can add up. Two tanks averaged as mpg give the wrong answer unless both covered exactly the same distance — add the miles, add the fuel, and divide once instead.
Fuel economy is distance divided by fuel: how far a car travelled on what it drank. The manufacturer publishes a laboratory figure for it, the dashboard displays a rolling estimate of it, and neither is the number your car returns on your roads with your right foot. This page takes the only measurement that belongs to you — the miles on the trip meter and the litres or gallons the pump dispensed — and turns it into a figure in every unit that matters at once.
It deliberately stops there. Money is a separate question with its own inputs, and mixing the two produces a figure that changes when the pump price changes even though the car has not altered at all. Economy describes the machine; price describes the week you bought fuel in.
The method is brim to brim. Top up until the nozzle clicks off, note the odometer or zero the trip meter, drive until the tank is well down, then refuel to the same click at the same pump. Distance since the last fill, divided by the fuel that went in, is the whole calculation — and the fuel that went in is what refilled the tank, not what you imagine it holds.
One tank is a noisy sample. Automatic cut-offs stop at slightly different levels, forecourts are not perfectly level, and a single motorway run flatters a car that spends the rest of its life in traffic. Three or four consecutive tanks, added together as one long trip, settle down into something you can quote. Partial fills are the classic mistake: half a tank and a guess about what remained turns a measurement back into an estimate.
A US gallon holds 3.785411784 litres. An imperial gallon holds 4.54609. Both are called a gallon, both give a number called mpg, and the imperial one is larger by a little over a fifth — so the same tank of fuel scores about 20% better when counted in imperial gallons than in US ones.
Take 300 miles on 10 gallons. Counted in US gallons that is 30 mpg; counted in imperial gallons the car covered the same 300 miles on 45.5 litres rather than 37.9, which is 25 mpg in US terms. Two figures, both honest, describing entirely different cars. That is why the output above is printed four ways with the unit attached to every one of them, and why a figure copied off a forum without its gallon is not evidence of anything.
Litres per 100 km counts fuel over a fixed distance, so the arrow points the opposite way: 5 is thrifty and 12 is thirsty. It is also the version that behaves itself arithmetically, because it is proportional to fuel burned. Halve it and you have halved the fuel.
That difference has a practical edge. Neither measure can be averaged across a journey unless every stretch happened to be the same length — take a motorway leg at 45 mpg and a short town leg at 20 mpg and the honest combined figure is nowhere near 32.5. Add the distances, add the fuel, and divide once. The tool follows that rule internally, which is why feeding it several tanks at a time gives a different and better answer than averaging the individual results.
The pump is. A trip computer infers consumption from injector timing and is calibrated optimistically on many cars, commonly by three to eight per cent. Track both for a few tanks and you will find your car has a consistent offset, which is useful to know.
At least three, and preferably ones that resemble your normal driving rather than a single long run. Treat them as one continuous trip: total the distance, total the fuel, divide once. A single tank can be several per cent out purely from where the nozzle cut off.
Not without converting. Beyond the gallon being a different size, the two are measured on different test cycles, so even the converted numbers describe different driving. Use them as a rough ordering of cars, never as a difference you could bank.
Yes, because the arithmetic only cares about volume of fuel and distance travelled. A plug-in hybrid is the exception worth flagging: its fuel figure alone ignores the electricity it drew from the wall, so it will look extraordinary and tell you very little.