Fuel Economy Calculator

A measurement, not a specification

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.

Getting a reading worth keeping

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.

Why every figure here names its gallon

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.

The conversion is a division, not a multiplication, and the constant depends on the gallon: 235.2146 divided by a US mpg figure gives litres per 100 km, and 282.481 divided by an imperial one does the same job. Both constants are exact ratios rather than measurements — the litre, the mile and both gallons are all defined quantities — printed here to the digits that matter. Every cell in the table is that arithmetic and nothing else.

The metric figure runs the other way

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.

Miles per gallon is a rate with the fuel underneath, which makes it deceptive at the economical end of the scale. Going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50 does, even though the second improvement looks twice the size. On the metric scale the same two changes read as 15.7 down to 11.8, against 5.9 down to 4.7 — and the first is plainly the larger saving.

Two legs cannot be averaged into one figure

Neither measure can be averaged across a journey unless every stretch happened to be the same length. Take 90 miles of motorway at 45 mpg and 10 miles of town at 20 mpg. The plain average of the two rates is 32.5 mpg. The honest answer is that the car used 2 gallons on the first leg and half a gallon on the second, covering 100 miles on 2.5 gallons, which is 40 mpg.

The plain average is out by more than seven miles to the gallon because it gives a ten-mile crawl the same weight as ninety miles of cruising. 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.