Fuel Efficiency Explained: MPG (US & UK), L/100km, km/L — and What They Cost You
Why the world measures fuel efficiency four different ways
Walk into a car dealership in Detroit, London, Berlin or Delhi and you will hear four completely different answers to the same question: “how much fuel does it use?” In the United States the answer comes in miles per gallon (MPG), in Britain it is also MPG but a meaningfully different one, across most of Europe it is litres per 100 kilometres, and in India, Japan and much of Asia it is kilometres per litre. A fuel efficiency converter exists because none of these numbers can be compared directly — and because one of them is a trap that has confused drivers and car reviewers for decades. This guide explains every unit, the exact maths behind each conversion, the notorious US vs UK gallon difference, how to rate any car's efficiency, and how to turn an efficiency figure into a real money number for your next road trip.
The four units, precisely defined
Miles per gallon (MPG, US) is distance travelled per US gallon of fuel. One US gallon is defined as exactly 3.78541 litres. Higher numbers mean better efficiency: a 35 MPG (US) car travels 35 miles on each US gallon it burns. This is the standard measurement on American window stickers and in US car reviews.
Miles per gallon (MPG, UK) is distance travelled per imperial gallon. One imperial gallon is exactly 4.54609 litres — about 20.1% larger than the US gallon. Because the gallon is bigger, the same car always scores a higher MPG figure in Britain than in America. A car that manages 35 MPG in the US manages about 42 MPG in the UK, with zero change in actual fuel consumption. This is the single most common source of confusion in fuel economy discussions online: a British reviewer praising a car's “50 MPG” is describing what an American would call roughly 41.7 MPG.
Litres per 100 kilometres (L/100km) is the volume of fuel consumed to travel 100 km, and it is the legal standard in the European Union, Australia, China and many other markets. Crucially, it works in the opposite direction from MPG: a lower number is better. A car rated at 5 L/100km is more efficient than one rated at 8 L/100km. The unit's big advantage is that fuel use scales linearly with it — double the L/100km and you exactly double the fuel bill for the same trip, which is not intuitive with MPG.
Kilometres per litre (km/L) is distance per litre of fuel and is the default in India, Japan, Brazil and much of Southeast Asia. It behaves like MPG — higher is better — but with metric units. Indian buyers in particular know it as “mileage”: when someone says a bike gives “45 ka mileage”, they mean 45 km/L.
The exact conversion maths (no approximations)
Every conversion between these units is a closed-form calculation — no lookup tables, no estimates. The master constant for US MPG is 235.214583: divide it by any US MPG figure and you get L/100km. So 35 MPG (US) equals 235.214583 ÷ 35 = 6.72 L/100km. The UK master constant is 282.480936: 42 MPG (UK) equals 282.480936 ÷ 42 = 6.73 L/100km — confirming that 35 US MPG and 42 UK MPG describe the same physical car. Between the two MPG standards the ratio is simply the ratio of the gallons: 1 UK gallon = 1.20095 US gallons, so multiply US MPG by 1.20095 to get UK MPG, or divide UK MPG by 1.20095 to get US MPG. Between km/L and L/100km the relationship is beautifully simple: they are reciprocals scaled by 100, so km/L = 100 ÷ L/100km. A car at 6.72 L/100km therefore does 14.88 km/L.
The US vs UK MPG difference: why it matters in the real world
The imperial gallon traces back to the British Weights and Measures Act tradition and was standardised at 4.54609 litres; the US gallon descends from the older English wine gallon at 3.78541 litres. When Britain kept selling fuel economy in MPG after metrication, the number quietly diverged from America's. The practical consequences are everywhere. Comparing a British and an American review of the same model without converting gives you a phantom 20% difference. Imported-car listings sometimes carry the wrong country's figure. And fuel-cost estimates built on the wrong gallon can be off by a fifth. Whenever you see an MPG number, your first question should be: which gallon? SipMile's converter labels every result explicitly — “MPG · US gallon” and “MPG · UK gallon” — so the ambiguity disappears.
How to rate any car's efficiency: the bands
Raw numbers mean little without context, so it helps to sort cars into bands. In L/100km terms — the unit engineers and regulators increasingly treat as the reference — a useful worldwide breakdown is: Excellent, 5 L/100km or under (full hybrids like the Toyota Prius, small efficient diesels, and the cost-equivalent of most EVs); Good, 5–7 L/100km (modern compact petrol cars, efficient crossovers); Average, 7–10 L/100km (typical mid-size sedans and family SUVs, where most new cars still sit); and Poor, above 10 L/100km (large SUVs, performance cars, pickups and anything built before efficiency became a selling point). In US MPG those bands are roughly: excellent ≥ 47 MPG, good 34–47, average 24–34, poor below 24. In UK MPG: excellent ≥ 56, good 41–56, average 28–41, poor below 28. And in km/L: excellent ≥ 20, good 14–20, average 10–14, poor below 10. Bear in mind that official laboratory figures are optimistic — real-world driving usually lands one band worse, especially for hybrids in cold weather or diesels in stop-start traffic.
EVs and plug-in hybrids: the same idea, different unit
Electric vehicles don't burn litres, so they are rated in kWh per 100 km — the exact electrical twin of L/100km. A typical modern EV consumes 14–18 kWh/100 km; an efficient one dips under 13. To compare an EV's running cost with a petrol car, convert the electricity to money: at $0.15 per kWh, 15 kWh/100 km costs $2.25 per 100 km — the same as a petrol car doing roughly 1.5 L/100 km at $1.50/L, deep in the Excellent band. That is why EVs cluster far beyond even the best hybrids on any efficiency scale. Plug-in hybrids are the awkward middle: their official figures assume frequent charging, so an unplugged PHEV often scores in the Average or Poor band in real use. If you are cross-shopping, run both the petrol and electric figures through the trip-cost calculator with your local fuel and electricity prices — the answer is rarely what the brochure implies.
From efficiency to money: the trip-cost formula
An efficiency figure becomes a budget the moment you multiply it by distance and price. The formula is: trip cost = (L/100km ÷ 100) × distance in km × price per litre. A 350 km road trip in a car doing 6.7 L/100km with fuel at $1.50/L burns (6.7 ÷ 100) × 350 = 23.45 litres and costs $35.18. The same trip in a 12 L/100km SUV costs $63.00 — nearly double, for the same journey. Two companion figures are worth knowing: cost per 100 km (L/100km × price per litre) is the cleanest way to compare cars' running costs, and cost per 100 miles is its American cousin. SipMile computes all three live, with an editable fuel price and currency symbol so the numbers match whatever is on the pump near you. Try this experiment: enter your car's real efficiency, then price your daily commute for a month. Most drivers are surprised twice — first by the monthly total, then by how much one band of efficiency improvement would save them in a year.
Seven habits that genuinely move your number
Conversions tell you where you stand; driving habits decide where you could be. The improvements below are measured, not folklore. 1. Slow down on highways: aerodynamic drag rises with the square of speed, so dropping from 130 to 110 km/h typically cuts fuel use 15–20% — often a full rating band. 2. Keep tyres at the placard pressure: under-inflation by 0.5 bar adds roughly 3% consumption and wears tyres faster. 3. Ditch roof boxes and racks when empty: they can add 10–25% drag at motorway speeds. 4. Combine short trips: a cold engine uses up to 50% more fuel for the first few kilometres; one warm trip beats three cold ones. 5. Anticipate traffic: smooth acceleration and coasting to red lights beat stop-start sprinting by 10–20% in town. 6. Cut idling: more than a minute of waiting? Switch off — modern starters handle it. 7. Service on schedule: clogged air filters and worn spark plugs quietly push cars a band worse. Note what is not on this list: premium fuel in a car that doesn't require it, and most “fuel-saving” gadgets, which independent tests consistently show do nothing.
A note on official figures vs your dashboard
Official ratings — EPA in the US, WLTP in Europe and the UK — are laboratory tests designed for fair comparison, not prediction. WLTP figures are generally 10–20% optimistic versus mixed real-world driving; EPA numbers are closer but still flattering for aggressive drivers. The most honest number is your own: fill the tank, reset the trip meter, drive normally, refill, and divide litres by hundreds of kilometres. Feed that figure into SipMile's converter and rating gauge — it is the number your wallet actually experiences, and it is the right input for the trip-cost calculator before any long journey.
Frequently Asked Questions
How do I convert MPG to L/100km?
Divide 235.214583 by your US MPG, or 282.480936 by your UK MPG. For example, 35 MPG (US) ÷ 235.214583 = 6.72 L/100km, and 42 MPG (UK) = 6.73 L/100km. The key step is knowing which gallon your MPG figure uses — US and UK gallons differ by about 20%. SipMile's converter does this instantly and labels both gallons explicitly.
What is the difference between US MPG and UK MPG?
A US gallon is 3.78541 litres while a UK (imperial) gallon is 4.54609 litres — the UK gallon is about 20% larger. The same car therefore always shows a higher MPG number in Britain than in America: multiply US MPG by 1.20095 to get the equivalent UK MPG. A British “50 MPG” car is roughly 41.6 MPG in US terms.
Is a higher or lower L/100km better?
Lower is better. L/100km measures fuel consumed per 100 km, so 5 L/100km beats 8 L/100km. This is the opposite of MPG and km/L, where higher numbers mean better efficiency. Because it scales linearly, L/100km is the easiest unit for comparing fuel costs: double the L/100km, double the fuel bill for the same trip.
What is considered good fuel efficiency for a car?
In L/100km: 5 or under is excellent (hybrids and efficient small cars), 5–7 is good, 7–10 is average, and over 10 is poor. That translates to roughly 47+ MPG (US) / 56+ MPG (UK) / 20+ km/L for excellent, and below 24 MPG (US) / 28 MPG (UK) / 10 km/L for poor. Real-world driving usually scores about one band worse than official laboratory figures.
How do I calculate fuel cost for a road trip?
Use: (L/100km ÷ 100) × trip distance in km × fuel price per litre. For a 350 km trip at 6.7 L/100km with fuel at $1.50/L, that's (6.7 ÷ 100) × 350 × 1.50 = $35.18. SipMile's trip-cost calculator does this live — enter your efficiency, edit the fuel price and currency, set the distance in km or miles, and it shows total cost plus cost per 100 km and per 100 miles.
How does EV efficiency compare to petrol cars?
EVs are rated in kWh/100 km — the electrical twin of L/100km. A typical EV uses 14–18 kWh/100 km, which at common electricity prices costs about the same to run as a petrol car doing roughly 1.5–2.5 L/100 km — far beyond the Excellent band. Plug-in hybrids only reach their advertised figures if charged regularly; driven unplugged they often land in the Average or Poor band.