How To Calculate Fuel Cost

Fuel cost for a journey comes from two multiplications and one division, and each of the three numbers you need has a natural home. The distance tells you how far you are going, the consumption figure tells you how much fuel each unit of distance demands, and the price tells you what that fuel costs. Multiplying distance by consumption and dividing by one hundred gives litres; multiplying litres by the price per litre gives the money. Everything else on this page is a consequence of those two steps and a careful look at the units.

Consumption is quoted in two incompatible styles, and they run in opposite directions. In much of Europe, Canada and Australia the figure is litres per 100 kilometres, where a smaller number means a more efficient car. In the United States and the United Kingdom the figure is miles per gallon, where a larger number means a more efficient car. Both describe the same physical behaviour, yet one improves as the other worsens, which is exactly why the pair is so easy to misread at a glance.

The relationship is a reciprocal, not a simple scaling. Converting 30 miles per gallon gives 235.214583 divided by 30, which is 7.8404861 litres per 100 kilometres; converting 7.5 litres per 100 kilometres gives 235.214583 divided by 7.5, which is 31.3619444 miles per gallon. The constant 235.214583 is not arbitrary: it is 100 kilometres divided by 1.609344 kilometres per mile, multiplied by 3.785411784 litres per US gallon. Because the two scales are reciprocal, a car that is twice as good on one scale is twice as good on the other, but a fixed improvement in litres per 100 kilometres becomes a shrinking gain in miles per gallon as the car gets more efficient.

Motorway driving usually returns lower consumption than stop-start town traffic, because a steady speed avoids the repeated acceleration that wastes energy and because an engine works most efficiently under a light, constant load. That pattern breaks down at high speed. Aerodynamic drag rises with the square of velocity, so pushing from 110 to 130 kilometres per hour adds far more than a proportional amount of resistance, and the engine must burn extra fuel to overcome it. Most cars therefore have a sweet spot somewhere in the middle of their range, often between 70 and 90 kilometres per hour, where consumption is lowest and either speeding up or crawling in traffic makes it worse.

The consumption figure printed in a brochure is measured under a standardised laboratory cycle, and real driving rarely reproduces it. Carrying passengers and luggage raises the mass the engine has to move; air conditioning draws power that ultimately comes from fuel; under-inflated tyres increase rolling resistance; and cold starts, headwinds, hills and congested traffic all push the number upwards. It is normal for real-world consumption to exceed the rated figure by ten to thirty per cent, and a loaded car on a hilly route can do considerably worse. Treating the rated value as a promise rather than an ideal is the first step towards an inaccurate budget.

Once you have a cost, the question is how to hold it. A figure per hundred kilometres is useful for comparing cars and consumption, but a figure per kilometre is far easier to carry in your head when you are planning. If a journey costs 0.12375 per kilometre, then a 30 kilometre commute is roughly 3.71 and a 450 kilometre trip is 55.6875 — the same rate applied to different distances. Cost per kilometre is also the only form that lets you compare a fuel-powered trip against a train fare, a taxi or a ride-share charge on equal terms, because those alternatives are usually priced per journey or per kilometre rather than per tank.

Unit discipline decides whether the answer is right. The distance must be in the same unit that the consumption figure assumes, so a car rated in litres per 100 kilometres needs a distance in kilometres, not miles. The price must match the volume unit, so a price per litre pairs with litres and a price per gallon pairs with gallons. Mixing a per-litre price with a volume computed in gallons, or feeding miles into a formula built for kilometres, produces a number that looks plausible and is quietly wrong by a large factor. Writing the units next to each number as you go is the cheapest way to catch this before it reaches a budget.

Fuel type and where you buy it matter as much as the car itself. Diesel, petrol and electricity are priced on different scales and carry different energy densities, so a diesel car with a lower consumption figure may still cost more per kilometre if diesel is priced higher at the pump. Prices also vary along a route, and a small detour to a cheaper station only pays off when the saving per litre exceeds the extra fuel the detour itself burns. Working that trade-off out is a direct application of the same formula, which is why it rewards anyone who has already done the arithmetic once.

The practical value of all this is comparison. Given the formula, you can ask whether a longer but faster motorway route beats a shorter stop-start one, whether a heavier load is worth the convenience, or whether a more efficient car justifies its price premium. Each question reduces to computing cost per kilometre under two sets of assumptions and subtracting. That is why the per-kilometre figure, rather than the headline consumption number, is the one worth remembering.