Anyone who’s ever compared fuel economy figures across different markets knows the frustration — a car rated at 30 mpg in the US suddenly looks like a gas guzzler in Europe, where the same metric is measured in litres per 100 kilometres. The catch is that “mpg” itself isn’t universal: the US gallon and the UK imperial gallon are different sizes, which means the same vehicle tells two very different stories depending on which side of the Atlantic you’re reading the label.

4 related posts

1 US MPG = 235.2 L/100km ·
1 UK MPG = 282.5 L/100km ·
1 L/100km = 235.2 US MPG ·
US conversion factor: 235.215 ÷ MPG ·
UK conversion factor: 282.481 ÷ MPG

Quick snapshot

1Confirmed facts
2What’s unclear
  • Which “good” threshold applies to your specific vehicle class
  • Real-world fuel economy often diverges from lab-test figures
3Timeline signal
  • EU and Canadian markets have used L/100km as standard since the 1990s
  • US EPA still publishes MPG; EU uses L/100km alongside CO2 ratings
4What’s next
  • Online converters can handle US or UK MPG selection for you
  • Mobile apps let drivers track real-world L/100km live
Label Value
US MPG to L/100km formula 235.215 ÷ MPG
UK MPG to L/100km formula 282.481 ÷ MPG
1 L/100km in US MPG 235.2
Average sedan L/100km 8–10
US gallon volume 3.785 litres
UK gallon volume 4.546 litres
1 mile 1.609 km

How do you convert MPG to L/100km?

The key thing to understand first is that your answer depends entirely on which gallon is being used. The US liquid gallon and the UK imperial gallon are different volumes — and since MPG measures distance per volume, a different volume means a different result even when the distance number stays the same.

US MPG formula

For US gallons, the formula is straightforward:

Formula

L/100km = 235.215 ÷ US MPG

This constant comes from dividing 100 by the number of kilometres in a mile (1.609344) multiplied by litres in a US gallon (3.785411784). According to mpgtolitres.com (fuel economy converter tool), 1 US MPG equals exactly 235.214583 L/100km — a figure that holds for any vehicle rated in American miles-per-gallon.

“235.215 ÷ 30 = 7.84 L/100km, which is a reasonable mid-range figure for a modern sedan.”

— Omni Calculator conversion reference

UK MPG formula

For UK or imperial gallons, the constant is larger because the imperial gallon holds more fuel:

Formula

L/100km = 282.481 ÷ UK MPG

Inch Calculator (conversion reference) confirms the UK conversion ratio is exactly 282.481. For example, 5 UK MPG works out to 56.4962 L/100km — a dramatically higher figure than the US equivalent because the imperial gallon contains 4.54609188 litres versus the US 3.785 litres.

“Using the imperial gallon formula, 40 UK MPG works out to 7.06 L/100km — still efficient, but notably higher than the US figure.”

— mpg-calculator.co.uk UK converter

Step-by-step conversion

To convert any MPG figure manually:

  1. Identify whether the MPG figure uses US or UK gallons. US listings typically just say “mpg”; UK listings may specify “imp mpg” or “UK mpg”.
  2. Select the correct constant: 235.215 for US, 282.481 for UK.
  3. Divide the constant by your MPG value.
  4. Round to two decimal places for practical use.

Confusing US and UK gallons is the most common conversion error. A 30 mpg car in the US is 7.84 L/100km; the same 30 mpg in UK terms would be 9.42 L/100km — a 20% difference that matters when cross-shopping imports.

What is 40 MPG in L/100km?

Forty mpg is a benchmark figure that many shoppers target when looking for a fuel-efficient vehicle. Here’s what that number looks like in both systems.

40 US MPG calculation

Calculation

235.215 ÷ 40 = 5.88 L/100km

According to mpgtokpl.com (conversion reference), the US formula derivation uses 100 ÷ ((mpg × 1.609) ÷ 3.785), which simplifies to the 235.215 constant. A 40 US MPG vehicle consumes just 5.88 litres to travel 100 kilometres — excellent by any global standard.

40 UK MPG calculation

Calculation

282.481 ÷ 40 = 7.06 L/100km

The same 40 mpg label in the UK translates to 7.06 L/100km — still efficient, but notably higher than the US figure. mpg-calculator.co.uk (UK-focused converter) confirms this is the result when using the imperial gallon formula.

Comparison table

Seven fuel economy benchmarks, shown in both US MPG and L/100km for easy cross-referencing:

Vehicle type US MPG L/100km Assessment
Hybrid sedan 50 4.70 Excellent
Efficient compact 40 5.88 Very good
Standard sedan 30 7.84 Average
Large SUV 22 10.69 Below average
Full-size pickup 18 13.07 Poor
Luxury performance 20 11.76 Average for class
Electric equivalent* 130 MPGe 1.81 Outstanding

*Electric vehicles in the US use MPGe (Miles Per Gallon Equivalent), which Omni Calculator notes converts to approximately 1.81 L/100km equivalent for comparison purposes.

What this means: a US buyer evaluating a UK-spec 40 mpg vehicle should remember that 7.06 L/100km is the real efficiency — about 20% worse than the same number suggests in North American terms.

What is a good L/100km fuel economy?

Context matters enormously here. A figure that signals excellence in a full-size pickup truck would be alarming in a subcompact. Below are benchmarks drawn from real-world and regulatory data to help you evaluate where any vehicle falls.

Benchmarks for cars

  • Under 5 L/100km: Outstanding — achievable by hybrids, plugin hybrids, and efficient diesels.
  • 5–7 L/100km: Very good — typical of modern high-efficiency petrol or diesel compacts.
  • 7–9 L/100km: Good — standard for most new petrol sedans and crossovers.
  • 9–11 L/100km: Average — common for larger sedans, midsize SUVs, and performance models.
  • Over 12 L/100km: High consumption — typical of large SUVs, full-size trucks, and performance vehicles.
The catch

Lab-test figures (WLTP in Europe, EPA in the US) consistently underestimate real-world consumption by 10–20%, according to mpgtokpl.com (conversion resource). Aggressive driving, extreme temperatures, and heavy loads can push real-world L/100km 25% higher than the window sticker. If you’re looking to check your vehicle’s insurance status in Ireland, you can Comprova l’assegurança del teu vehicle a Irlanda.

Is 9.2 L/100km good?

Yes — 9.2 L/100km translates to roughly 25.6 US MPG, which is the average for a new US sedan according to Omni Calculator. In the UK, that’s about 30.7 UK MPG. It’s neither frugal nor wasteful by modern standards — solid middle-of-the-road efficiency.

Is 11.5 L/100km good?

Eleven point five litres per 100 kilometres works out to roughly 20.5 US MPG. That’s below the average for new vehicles and would flag a large SUV or pickup truck. For a vehicle in that class, it’s acceptable; for a sedan or hatchback, it signals a relatively thirsty engine.

Is 10.7 L/100km good?

At 10.7 L/100km you’re looking at approximately 22 US MPG. mpg-calculator.co.uk (UK converter tool) puts this in context as a typical figure for a midsize SUV or a larger engine compact. Reasonable for the class, but not efficient.

Is 5.7 L/100km good?

Absolutely — 5.7 L/100km equals about 41.3 US MPG, firmly in hybrid territory. Inch Calculator (conversion reference) confirms this is the kind of figure achieved by Toyota Hybrids, efficient diesel compacts, and some plugin hybrid models on battery-assist driving.

Bottom line: “Good” is relative to vehicle class. A sedan doing 10.7 L/100km is underperforming; a full-size pickup doing the same is doing fine. Always compare like to like.

How do you convert UK MPG to litres per 100km?

UK drivers and anyone importing a vehicle registered in the United Kingdom need to work with the imperial gallon — and that small difference in gallon size ripples through the entire calculation.

UK imperial gallon differences

The imperial gallon holds 4.54609188 litres, compared to the US gallon’s 3.785411784 litres. That’s roughly 20% more volume per gallon. Since MPG measures distance per unit of fuel, the larger UK gallon means the same numerical mpg value represents better fuel efficiency in the imperial system. As Omni Calculator notes, 1 UK MPG = 0.8327 US MPG — and conversely, 1 US MPG = 1.201 UK MPG.

Formula and examples

UK formula

L/100km = 282.481 ÷ UK MPG

Working through the math: a UK Ford Focus rated at 50 UK MPG consumes 282.481 ÷ 50 = 5.65 L/100km. In the US, that same vehicle would be listed at about 60 US MPG. Inch Calculator (conversion reference) validates this cross-system consistency using the 282.481 constant.

Common pitfalls

  • Assuming mpg is universal: Online listings that just say “mpg” without specifying US or UK may confuse buyers. Always check the vehicle’s registration country.
  • Mixing up conversion direction: The reciprocal formulas (L/100km to MPG) are different for each system. According to My Math Tables (math reference): US MPG = (100 × 3.785) ÷ (1.609 × L/100km) and UK MPG = (100 × 4.546) ÷ (1.609 × L/100km).
  • Ignoring real-world deviation: Both lab-test systems (EPA and WLTP) produce numbers that diverge from real-world driving by different margins.

The implication: a US buyer evaluating a UK-import vehicle who uses 235.215 instead of 282.481 will underestimate fuel consumption by about 20% — enough to misclassify a vehicle as “good” when it’s actually average.

What is a good MPG?

MPG benchmarks also depend on which gallon system is in use, but the underlying principle is the same: higher miles per gallon means less fuel burned per mile. Here’s how to evaluate MPG figures from either side of the Atlantic.

Average car MPG benchmarks

  • 30+ US MPG: Good for petrol sedans; above average for the US market.
  • 40+ US MPG: Very good; typical of hybrids, efficient turbocharged compacts.
  • 50+ US MPG: Excellent; achievable by full hybrids and some efficient diesels.
  • 40+ UK MPG: Good for petrol; decent for the UK market average.
  • 50+ UK MPG: Very good; common for efficient diesels and hybrids.

US vs UK standards

Because the US and UK gallons differ, a direct MPG comparison requires converting both to L/100km first. Omni Calculator (conversion tool) provides this comparison function directly, allowing side-by-side evaluation without manual calculation. Without conversion, a UK-listed 40 mpg vehicle looks less efficient than a US-listed 40 mpg vehicle — but that’s a measurement artefact, not a real difference.

Relating MPG to L/100km

To move between the two systems quickly:

Upsides

  • L/100km is more intuitive for metric countries — lower number means less fuel
  • MPG is familiar and familiar to US/UK buyers — higher is clearly better
  • Conversion formulas are exact and consistent
  • Online converters eliminate manual calculation errors

Downsides

  • US vs UK MPG confusion causes real errors when buyers cross-shop internationally
  • Lab-test MPG figures diverge from real-world consumption by 10–25%
  • MPG improvements are non-linear — going from 20 to 25 MPG saves more fuel than 40 to 45 MPG
  • Electric vehicles use MPGe, which doesn’t map cleanly to either system
The trade-off

MPG penalises efficient vehicles with an inflation effect: a 1 MPG improvement at 15 MPG saves far more fuel than a 1 MPG improvement at 45 MPG. L/100km avoids this quirk — a 1 L/100km improvement means the same absolute fuel saving at any point on the scale. This is why mpgtokpl.com (conversion resource) notes that regulators in most countries have settled on L/100km as the fairer metric for consumer communication.

Bottom line: US buyers looking at European imports should target under 6 L/100km for an equivalent “good” rating; UK buyers looking at US imports should treat anything over 35 US MPG as genuinely efficient. Converting both to L/100km removes the confusion entirely.

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Frequently asked questions

What does MPG stand for?

MPG stands for Miles Per Gallon — the number of miles a vehicle can travel on one gallon of fuel. It’s the standard fuel economy measure in the US and UK, though the two markets use different gallon sizes, which means identical numerical mpg figures represent different real-world efficiencies.

Why use L/100km over MPG?

L/100km (litres per 100 kilometres) is the standard in most metric countries including the EU, Canada, and Australia. Its key advantage over MPG is that lower numbers always mean better efficiency — it’s linearly intuitive. MPG has a non-linear quirk where improvements at low MPG values save more fuel than equivalent improvements at high MPG values, which can mislead consumers.

How accurate are online converters?

Most reputable online converters use the established constants: 235.215 for US MPG and 282.481 for UK MPG. As long as you select the correct gallon type (US or imperial), the result will match what you’d get with a scientific calculator. Sources like mpgtolitres.com and Omni Calculator confirm these constants to six significant figures. The only source of error is selecting the wrong gallon type.

Does fuel type affect conversions?

No — the MPG to L/100km conversion is purely a mathematical relationship between distance and volume. Fuel type (petrol, diesel, electricity) doesn’t change the formula. However, fuel type does affect real-world MPG: diesel engines typically achieve 20–30% better MPG than petrol equivalents, and electric vehicles use MPGe (Miles Per Gallon Equivalent) which converts using energy content rather than physical volume.

What is average truck MPG?

Full-size pickup trucks in the US typically average 17–22 US MPG, depending on engine size and drivetrain (2WD vs 4WD). Converted to L/100km using the 235.215 constant: 13.8–10.7 L/100km for a 17–22 MPG truck. Diesel-powered trucks improve this to around 22–26 US MPG, or 10.7–9.0 L/100km. As mpg-calculator.co.uk (UK converter tool) notes, the gap between US and UK MPG figures for the same truck can be 20% due to the gallon difference.

How to measure real-world fuel economy?

Fill your tank to the brim, note the odometer reading, drive your normal route, then fill up again and note the litres pumped. Divide the kilometres driven by the litres used, then multiply by 100: (km driven ÷ litres used) × 100 = real-world L/100km. Compare this to the window-sticker figure — a gap of more than 15% suggests aggressive driving habits, tire issues, or a maintenance problem worth investigating.

Are there apps for MPG to L/100km?

Yes — most major fuel-tracking apps support both MPG and L/100km units, allowing you to switch displays without recalculating. My Math Tables (math reference) and Inch Calculator both offer free tools. Many nav apps and dashcam systems now include fuel economy logging as a standard feature with automatic unit conversion.

For drivers comparing vehicles across markets, the conversion from MPG to L/100km is a one-time calculation — but getting the right constant matters. A US buyer evaluating a UK-spec vehicle who uses 235.215 instead of 282.481 will underestimate fuel consumption by about 20%. The formulas are exact; the only error is in forgetting which gallon is being measured.