Understanding L/100 km and what a trip costs

A rate of consumption, not a score — which is why lower wins

Litres per 100 kilometres answers a specific question: to travel a fixed distance, how much fuel do you have to buy? A car rated at 7.5 L/100 km needs 7.5 litres to cover 100 km. One rated at 11 L/100 km needs 11 litres for the same distance. Less fuel for the same journey is better, so the smaller number is the better car.

That is the reverse of every other fuel figure in common use. Kilometres per litre and miles per gallon both put distance on top: they ask how far you get from a fixed amount of fuel, so bigger is better. Both conventions describe exactly the same physical thing, and each is the other turned upside down — which is precisely why people misread them. Someone used to mpg reads “7.5” as terrible and “14” as excellent, when in L/100 km it is the other way round.

The Australian convention has a genuine advantage beyond habit. Because L/100 km puts fuel on top, it is directly proportional to litres burnt and therefore to dollars spent. A one-unit improvement is worth the same amount of fuel wherever you start from: dropping from 12 to 11 L/100 km saves exactly as much fuel per 100 km as dropping from 6 to 5. Miles per gallon does not behave that way at all — going from 20 to 21 mpg saves far more fuel than going from 40 to 41 — which is why mpg comparisons routinely mislead about where an efficiency gain actually matters.

Converting between L/100 km, km/L and both kinds of mpg

The conversion to kilometres per litre is a reciprocal. The conversions to miles per gallon are the same reciprocal with a constant folded in, and there are two different constants because there are two different gallons still in use.

km/L         = 100 ÷ (L/100 km)
US mpg       ≈ 235.215 ÷ (L/100 km)
imperial mpg ≈ 282.481 ÷ (L/100 km)

Those constants are not arbitrary. A mile is 1.609344 km and a US gallon is 3.785411784 litres, and combining them gives 100 ÷ 1.609344 × 3.785411784 = 235.215. The imperial gallon is 4.546090 litres, which produces 282.481 the same way. The imperial figure is about 20% larger than the US one for identical fuel use, so an unlabelled mpg number is genuinely ambiguous: British and Australian publications historically meant imperial gallons, American ones mean US gallons.

Notice how compressed the low end of the L/100 km scale is and how stretched the mpg column becomes. From 4.0 to 5.5 L/100 km is a 1.5-unit step and a 16 mpg gap; from 11.0 to 14.0 is a 3-unit step and only a 4.6 mpg gap. Same reciprocal, wildly different visual impression. If you are comparing two cars, do it in the units you will be paying in.

Measuring your own consumption, brim to brim

Your car has three fuel consumption figures: the one on the label, the one on the dashboard, and the one you are actually paying for. Only the third is yours, and measuring it takes one tank of fuel and a trip meter.

1. Fill the tank until the pump nozzle cuts off, and do not top up beyond that. The cut-off point is your reference level, so it needs to be repeatable.
2. Reset the trip meter to zero at the pump, before you drive away.
3. Drive normally. Do not adjust your driving for the measurement, or you will measure a car you do not own. A full tank is a better sample than a couple of hundred kilometres.
4. Refill to the same cut-off point, ideally at the same style of pump, and note the litres shown on the pump and the kilometres on the trip meter.
5. Divide litres by kilometres and multiply by 100. For example, 58.2 litres over 712 km is 58.2 ÷ 712 × 100 = 8.17 L/100 km.
6. Repeat over two or three tanks and average. A single tank can be out by a few per cent purely from where the nozzle cut off.

This beats the trip computer because it measures what the pump actually delivered rather than estimating from how long the injectors were open. Dashboard readouts are usefully responsive for seeing the effect of a heavy right foot in real time, but they are commonly a few per cent optimistic, and their long-term average silently resets in ways that are easy to miss. Where the two disagree, the fuel receipt is the one that matches your bank statement.

It is worth measuring separately for the driving you actually do. A tank consumed on a highway run and a tank consumed on school drop-offs will produce meaningfully different figures for the same car, and averaging them together describes neither trip.

Why nobody matches the windscreen figure

New vehicles sold in Australia carry a fuel consumption label showing litres per 100 kilometres, usually broken into urban, extra-urban and combined figures. Those numbers come from a standardised laboratory test cycle run under controlled conditions, and the Green Vehicle Guide is where the official figures for a given model are published.

The point of a standardised test is comparability, not prediction. Every car is measured the same way, so the label is a fair basis for ranking two models against each other. It is not a forecast of your commute, because your commute has traffic lights, hills, cold starts, a load in the back and a driver in a hurry. Expecting to match a laboratory figure is the same category of error as expecting to match an advertised broadband speed at peak hour.

• Urban versus highway. Stop-start driving is the most expensive kind, because accelerating a mass repeatedly and then throwing the energy away as brake heat is inherently wasteful. Steady highway cruising is usually a car’s best figure, until speeds get high enough for aerodynamic drag to dominate.
• Cold starts and short trips. An engine that has not reached operating temperature runs richer and less efficiently. A 3 km trip can easily use double the per-kilometre fuel of the same route once warm.
• Load and towing. Extra mass costs fuel on every acceleration and every hill. Towing changes the figure dramatically, not marginally.
• Aerodynamics. Roof racks, roof boxes and bikes on the roof add drag that grows with the square of speed, which is why they cost far more at 110 km/h than around town. Removing an empty rack is free efficiency.
• Tyre pressure. Underinflated tyres increase rolling resistance. Checking pressures against the placard is the cheapest consumption improvement available.
• Air conditioning. The compressor is a real load, most noticeable at low speeds where the engine is producing little power anyway.
• Driving style. Hard acceleration, late braking and higher cruising speeds all add up. The same car and route can vary by well over 20% between drivers.

Costing a drive before you leave

Once you have a consumption figure you trust, costing a journey is two multiplications. Convert the distance into hundreds of kilometres, multiply by the consumption to get litres, then multiply by the price at the pump.

litres = distance in km ÷ 100 × (L/100 km)
cost   = litres × price per litre

Two cautions on the price. Pump prices are GST-inclusive, so there is no GST to add. And they move on a weekly cycle in most capital cities, which means the price you fill at on departure and the price you find halfway across a state can differ by 20 cents a litre or more. For a long drive, costing at a slightly pessimistic price is more useful than costing at the cheapest figure you have seen.

Fuel is also not the whole cost of a drive. Tolls, parking, tyres, servicing and depreciation are real and none of them appear in a litres-times-price calculation. If you are working out whether to drive or fly, or reconciling a travel budget, Moneysmart’s budgeting guidance is a reasonable frame for the costs that sit around the fuel.

Why Australian figures do not line up with American reviews

Australia completed its conversion to the metric system in the 1970s, and fuel consumption settled on litres per 100 kilometres — litres and kilometres both being SI-derived units, and the per-100-km form being the European convention Australia followed. American reviews are written in US miles per gallon, older British material in imperial miles per gallon, and a handful of markets still quote km/L.

So a US review praising a car for “32 mpg combined” is describing roughly 7.35 L/100 km, which is unremarkable by Australian standards. A British review quoting the same 32 mpg in imperial gallons is describing about 8.83 L/100 km, which is noticeably thirstier. Nothing about the car changed; only the gallon did. Convert to L/100 km before forming an opinion, and if the article does not say which gallon it means, treat the number as unusable.

Distances have the same problem in a milder form. A road-trip figure quoted in miles needs converting before it can be combined with an L/100 km consumption, and the length converter handles that without the mental arithmetic.

What the OzCalc fuel calculators model

Three tools cover this ground, and each deliberately does one thing. The fuel economy converter moves between L/100 km and km/L — the two ways of writing the same consumption. The fuel cost calculator takes a distance, a consumption figure and a price per litre and returns the litres needed, the trip cost, the return cost and the cost per kilometre. The travel time calculator estimates how long the drive takes from distance and an average speed.

The simplifications matter, because they are the difference between an estimate and a plan.

• Consumption is treated as constant for the whole trip. A drive that is half city and half highway is better costed as two calculations with two different figures.
• The distance is what you enter. Detours, wrong turns, and the urban driving at each end of a highway run are not included, and a return trip is simply the one-way distance doubled.
• Idling, traffic and hills are invisible. So are headwinds, roof racks and a full boot.
• The price is a single figure. Refuelling partway at a different price means splitting the trip or using a weighted average.
• Travel time uses an average speed, not live traffic, road conditions or rest breaks — so it is a lower bound on a long drive rather than an arrival time.

The one input worth getting right

Of distance, consumption and price, consumption is the input people are least sure about and the one that most changes the answer over a long trip. Spending one tank of fuel on a brim-to-brim measurement replaces a laboratory figure with your own, and every trip estimate afterwards inherits the improvement.

Common questions