How this is worked out
Litres and kilometres agree with each other, so the calculation is direct:
litres = distance in km × L/100km ÷ 100
cost = litres × price per litre
500 km at 8.5 L/100km is 42.5 litres, and at $1.85 a litre that is $78.63 — about 15.7 cents a kilometre. A car with four people aboard splits that to $19.66 each, which is the calculation that decides most road trips.
Conversions are only needed for numbers from overseas, and then you need two, because "MPG" means different things in America and Britain:
MPG (US) = 235.215 ÷ (L/100km) · MPG (imperial) = 282.481 ÷ (L/100km)
8.5 L/100km is 27.7 MPG (US) and 33.2 MPG (imperial). The constants fall out of the exact definitions — 3.785411784 litres to a US gallon, 4.54609 to an imperial gallon, 1.609344 km to a mile.
A worked example
- Trip distance
- 500 km, one way
- Fuel consumption
- 8.5 L/100km
- Fuel price
- $1.85 a litre — a placeholder, not a quote
- Fuel used
- 42.50 litres
- Trip cost
- $78.63
- Cost per kilometre
- $0.16 — $15.73 per 100 km
- Split four ways
- $19.66 each
- 13,000 km a year
- $2,044.25 — $170.35 a month
- The same distance at 6.5 L/100km
- $1,563.25 a year — $481.00 saved
- 8.5 L/100km is also
- 27.7 MPG (US) · 33.2 MPG (imperial) · 11.76 km/L
Distances are long here, so cents per kilometre compound
Sydney to Melbourne is about 880 km, Perth to Adelaide about 2,700 km, and Brisbane to Cairns about 1,700 km. At 15.7 cents a kilometre in fuel, that Sydney–Melbourne run is roughly $138 each way, and the Perth–Adelaide crossing is well over $400 in fuel alone before anyone has eaten or slept.
That scale is what makes small differences in consumption matter. A litre per 100 km either way is worth about $1.85 per 100 km travelled, which sounds trivial until it is multiplied by 13,000 km a year and then by the years you keep the car.
It is also why the return-trip checkbox is worth using. Half of all trip-cost errors are simply forgetting that you have to come home, and on these distances that is not a rounding error.
The label figure, and the figure you will actually see
New cars carry a fuel consumption label with city, highway and combined figures from a standardised laboratory test. It is a fair basis for comparing two cars tested the same way, and a poor prediction of what you will get, because the test has no roof racks, no towbar, no 40-degree day with the air conditioning at full, and no gravel.
Towing and load change consumption more than almost anything else. A caravan can add half again to a tow vehicle's consumption; a roof pod costs several per cent at highway speed; underinflated tyres and a full tank of luggage cost a little more. Air conditioning in an Australian summer is not optional and is not free.
The reliable approach is to measure. Fill, zero the trip meter, drive, fill again, and divide litres by the distance in hundreds of kilometres. Enter that figure here rather than the label, and the annual number will be much closer to your bank statement.
Two different MPGs, and neither of them is yours
Australia has used L/100km since metrication, but most car reviews Australians read are American or British, and both quote MPG. They are not the same MPG. The US gallon is 3.785411784 litres and the imperial gallon is 4.54609 — 20.1% apart — so the same car is 27.7 MPG in an American review and 33.2 MPG in a British one.
Older Australians and older workshop manuals sometimes still use miles per imperial gallon, which is the British figure, because that is what was used here before 1974. If you have inherited a number from that era it is the larger of the two.
The table under the result converts whatever you type into all four units at once, which turns "is 33 MPG good?" into a question you can answer by looking at the L/100km line beside it.
Lower is better, and equal steps are not equal savings
L/100km is a lower-is-better scale, and it has a real advantage over MPG beyond direction: differences on it are proportional to fuel used. Over 13,000 km, dropping from 10 to 9 L/100km saves 130 litres, and dropping from 6 to 5 saves the same 130 litres. On the MPG scale those two identical savings look completely different.
That is why replacing a thirsty vehicle usually saves far more than upgrading an already-efficient one, even when the second upgrade looks more impressive as a percentage. If you run a ute at 11 L/100km and a small car at 6, a litre off the ute is worth more than a litre off the car in absolute fuel — and both are worth exactly $1.85 per 100 km at today's placeholder price.
The comparison panel prices both vehicles over the same annual distance so the decision is made in dollars, which is a scale where the arithmetic behaves. Remember it is fuel only: registration, insurance, servicing, tyres and depreciation are all real and none of them appear on a fuel receipt.
Assumptions and sources
- US gallon
- 3.785411784 litres exactly (231 cubic inches).
- Imperial gallon
- 4.54609 litres exactly — the 20.1% gap between the two MPG scales.
- Mile
- 1.609344 kilometres exactly, international yard and pound agreement 1959.
- Conversion constants
- MPG (US) = 235.215 ÷ L/100km; MPG (imperial) = 282.481 ÷ L/100km, derived from the exact definitions rather than tabulated.
- Fuel price
- The default here is a PLACEHOLDER, not a researched figure. Capital-city prices move through a discount cycle and regional prices are consistently higher.
- Consumption labels
- Combined figures from the standardised test cycle on the new-vehicle fuel consumption label. Real-world consumption is typically higher, and much higher when towing.
- Arithmetic
- tripCost, annualFuel and efficiencyIn in assets/js/tools/work.js, verified in tools/test/work.mjs.