Fuel consumption is the only everyday quantity the world insists on measuring in two opposite directions. Litres per 100 kilometres counts fuel per distance, so smaller is better. Miles per gallon counts distance per fuel, so bigger is better. Both are honest; they are not equally intuitive, and confusing them has embarrassed many a traveller doing petrol-station maths in a rented car. This article explains both units, converts between them cleanly, and shows which one behaves better for trip planning.
One more unit haunts the edges — kilometres per litre, used in parts of Asia — which behaves like mpg (bigger is better) and converts by division into 100. Three dialects for one quantity is two too many, and the article’s sympathy lies with whichever one your dashboard speaks, translated carefully.
Why the two directions exist
L/100 km descends from engineering: it answers ‘how much fuel must I carry for a known journey?’ — a linear, plannable quantity. MPG descends from the forecourt: ‘how far will this gallon take me?’ — the question of a driver buying fuel in fixed coins. Neither camp is wrong; they optimise different questions. But note the asymmetry: consumption per distance adds up across a trip and averages cleanly, while distance per fuel does neither, because gallons do not divide journeys evenly.
Key takeaways
- L/100 km: smaller is better, and it averages honestly
- MPG: bigger is better, and it misleads when averaged
- Convert: L/100 km = 282.5 ÷ MPG (imperial)
The MPG illusion
The danger of mpg is its non-linearity, documented by researchers who named it the ‘MPG illusion’. Upgrading a car from 15 to 20 mpg saves far more fuel than upgrading from 35 to 40 mpg over the same distance — yet the improvements look identical on the badge. In consumption terms the trick is visible instantly: the first jump is 15.7 → 11.8 L/100 km (saving 3.9), the second is 6.7 → 5.9 (saving 0.8). Fleet decisions and family car choices both go wrong here, in the same direction: mpg flatters improvements at the efficient end and hides them at the thirsty end, which is precisely backwards for saving fuel.
Converting without tears
With imperial gallons (the UK standard, 4.546 litres), the conversion is a single constant: L/100 km = 282.5 ÷ mpg, and mpg = 282.5 ÷ L/100 km. US gallons are smaller, so the constant changes to 235.2 — using the wrong one is a 20% error wearing a plausible face, and it is the commonest conversion accident in motoring. Our fuel calculator takes either unit and works internally in litres, asking for price per litre as pumps now almost universally display it.
| L/100 km | MPG (imperial) | MPG (US) |
|---|---|---|
| 5.0 | 56.5 | 47.0 |
| 6.5 | 43.5 | 36.2 |
| 8.0 | 35.3 | 29.4 |
| 10.0 | 28.2 | 23.5 |
| 12.0 | 23.5 | 19.6 |
Reading a hire car abroad
The practical exam for all of this is a foreign dashboard: a hire car reporting 6.2 (of something) while the fuel gauge drops and the next town is 140 km away. The workflow: identify the unit from the handbook screen or the magnitude — consumption figures sit between 4 and 15 L/100 km, or 15 and 60 mpg, or 8 and 25 km/L; the ranges barely overlap, so the number itself usually confesses its unit. Then compute range from the tank: litres in tank ÷ consumption × 100 km. Our fuel calculator runs this backwards and forwards in both unit systems, and it works offline once loaded — which, at a petrol station in a foreign valley, is not a trivial feature.
The same skill defends you at home: a seller quoting consumption in US mpg for a European car is inflating its efficiency by 20% without lying once.
Consumption as a diagnostic
Beyond planning, your car’s consumption is a health signal. A long-term average that creeps upward by 10–15% without a change in driving pattern is the car reporting something: under-inflated tyres, a dragging brake, a tired sensor, alignment out after a pothole. Tracking the figure — even crudely, fill-up to fill-up — turns the fuel calculator’s input into an early-warning instrument. Drivers who know their car’s number catch faults months before drivers who only know that it ‘seems thirstier lately’.
Record it the simple way: litres added and distance since the last fill, divided out. Three fill-ups average away the pump-to-pump noise and leave the truth.
Electric cars and the unit that replaced both
The feud in this article is quietly being ended by electricity. EVs are rated in kWh per 100 km (Europe) or miles per kWh (the Anglophone habit) — the same two directions, reincarnated, with the same illusion waiting: miles per kWh flatters improvements at the efficient end exactly as mpg did. The planning virtues remain with consumption-per-distance, and the conversion between the dialects is the first thing an EV driver crossing a border learns. The deeper continuity is that energy per distance is the honest currency of travel in any technology, because journeys are made of distance and tanks — or batteries — are made of energy.
Which suggests the epitaph for the whole dispute: the unit that wins is the one that matches the question. ‘What will this journey cost?’ wants consumption. ‘How far can I go?’ wants the reciprocal. Compute in the first, glance in the second, and never average the second — the rule this article has been teaching in two fuels now.
Which unit for trip planning?
For planning, consumption wins and it is not close: multiply litres-per-100 by distance in hundreds, multiply by price, and you have the trip cost — the exact chain this site’s fuel calculator automates, with round trips and passenger splits added. MPG can stay on the dashboard as a matter of taste. But when comparing two cars, two routes or two speeds, translate to consumption first and let the linear unit do the thinking.
Common mistakes
- Averaging mpg figures across a trip with different legs
- Using the US constant for a UK car (or vice versa)
- Comparing brochure figures measured on different test cycles
Questions this raises
They are measured on standard laboratory cycles for comparability, not prophecy. Real driving typically uses 10–25% more; your own long-term average from the trip computer or fuel receipts is the figure to plan with.
Standing still consumes fuel over zero distance, which is why very urban driving destroys consumption figures. It is also why a ‘per hour’ figure exists for stationary engines — not needed for trip planning on this site.
Put a number on it.
One unit rises when the car improves; the other falls. Why Europe and America measure thirst in opposite directions, and how to convert without tears.
Open the Fuel Cost Calculator