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Fuel Economy Converter

Convert between MPG (US), MPG (UK), L/100km and km/L with inverse relationship handling.

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Note: MPG and km/L measure distance per fuel (higher = more efficient), while L/100km measures fuel per distance (lower = more efficient). The conversion between these is inverse, not linear.

Using the Fuel Economy Converter

  1. Pick the source unit - MPG (US), MPG (Imperial / UK), liters per 100 kilometers, or kilometers per liter - from the "From" selector.
  2. Enter a number. The output updates on every keystroke through a Preact onInput handler.
  3. Pick the target unit in "To". Because the relationship between distance-per-fuel and fuel-per-distance is reciprocal (not linear), the tool inverts automatically when you cross that boundary.
  4. Swap with the arrow button to reverse direction.
  5. Copy the output via the Clipboard API.

Why the Conversion Is Not a Simple Multiply

Fuel economy can be expressed two fundamentally different ways: distance per unit of fuel (MPG, km/L - bigger is better) or fuel per unit of distance (L/100km - smaller is better). Going between those two involves a reciprocal, not a scale factor. The base identity is L/100km = 235.2145833... / MPG_US, derived from 1 US gallon = 3.785411784 L (exact, per NIST SP 811) and 1 mile = 1.609344 km (exact, per the 1959 international agreement). For Imperial MPG the constant becomes L/100km = 282.4809363... / MPG_UK because 1 UK imperial gallon = 4.54609 L exactly (UK Weights and Measures Act 1985). The reciprocal relationship means differences at the low end matter far more than at the high end: going from 10 to 20 MPG saves as much fuel as going from 30 to 60 MPG, a point the EPA emphasizes on its fueleconomy.gov website.

When You Reach For It

  • Comparing a European car review written in L/100km to US EPA labels quoted in MPG.
  • Translating an Imperial MPG figure from a UK magazine into US MPG for an American buyer.
  • Sanity-checking a hybrid\'s claimed 55 MPG (US) against the equivalent 4.3 L/100km.
  • Budgeting fuel for a road trip - km/L combined with a distance gives liters, which paired with a local fuel price gives cost.
  • Evaluating a car maker\'s NEDC vs WLTP ratings (both metric) against an EPA combined rating (US MPG).
  • Reading truck or fleet specs that mix L/100km (primary) and MPG_UK (legacy) on the same spec sheet.

US Gallon vs Imperial Gallon - Where the 20% Gap Comes From

The US gallon (3.785411784 L) and the Imperial gallon (4.54609 L) are two different units that share a name. The US gallon descends from the English wine gallon of 1707 (231 cubic inches); the Imperial gallon was redefined in 1824 to equal 10 pounds of distilled water at 62 F. The US kept the wine gallon. The two differ by about 20.1% (Imperial being larger), and because MPG is miles per gallon, the same car covers more miles per Imperial gallon than per US gallon. A car rated 30 MPG (US) equals 36.0 MPG (UK) and 7.84 L/100km. UK government fuel-economy publications and many UK-sold cars still use Imperial MPG; British drivers buy fuel in liters at the pump, which is mildly inconvenient. Always check the small print on any UK fuel-economy figure to see which gallon the author meant.

Pitfalls and Test Cycles

Three pitfalls matter beyond unit arithmetic. First, test cycles: the US EPA uses FTP-75 plus HWFET plus US06 for combined ratings; Europe uses WLTP (since 2017, replacing the older, rosier NEDC); Japan uses JC08. Ratings are not comparable across cycles - a car rated 40 MPG EPA combined may score 45 MPG on NEDC and 38 MPG on WLTP. Second, fuel density varies: diesel is about 12% more energy-dense than gasoline per liter, so a 7 L/100km diesel and a 7 L/100km gasoline car burn different amounts of CO2. Third, electric-vehicle efficiency is quoted in kWh/100km or MPGe (MPG-equivalent), which this converter does not handle; a dedicated EV calculator is the right tool. The tool does unit arithmetic only.

Alternatives

The EPA fueleconomy.gov website has a native L/100km to MPG widget and cross-references test-cycle data. For vehicle shopping, manufacturer spec sheets usually list both MPG and L/100km already - no conversion needed. For fleet analytics, Fleet Management Systems aggregate per-trip consumption with cost and route overlays, which this converter does not do. Python\'s pint library handles the reciprocal relationship in chained calculations. This page is the fastest answer when you have a single figure and need the other representation, with no dependencies and no server round-trip.

Frequently Asked Questions

What is the exact factor between MPG and L/100km?

L/100km = 235.2145833... / MPG_US, where the constant comes from (3.785411784 L/gal) * (100 km) / (1.609344 km/mi). For Imperial MPG the constant is 282.4809363... = (4.54609 L/gal_UK) * 100 / 1.609344. Both constants are exact-rational under the treaty definitions of the gallon and the mile; the decimal expansions are long because the underlying ratios are not short fractions.

Why does a better MPG number mean less fuel saved at the top end?

Because MPG and L/100km are reciprocals. Driving 10,000 miles in a 20 MPG car uses 500 gallons; in a 40 MPG car, 250 gallons - a 250 gallon saving. But going from 40 to 50 MPG saves only 50 gallons over the same distance. The EPA recommends thinking in gallons-per-100-miles (the US equivalent of L/100km) precisely because reciprocals distort the intuition about "incremental improvements".

Is my input sent to a server?

No. The conversion runs inside the Preact component in your browser - a few multiplications and one reciprocal, nothing more. There is no fetch call for the numbers and no websocket. PostHog records the pageview for aggregate usage stats but does not capture your input. The page works offline after the initial load.

How do I convert from US MPG to UK MPG?

Multiply by the gallon ratio: 1 Imperial gallon / 1 US gallon = 4.54609 / 3.785411784 = 1.20095042... So US MPG * 1.20095 gives UK MPG. A 30 MPG_US car is 36.0 MPG_UK. The opposite direction: UK MPG * 0.83267 = US MPG. UK used-car adverts almost always quote MPG_UK; import it and convert before comparing against EPA labels.

What is a good fuel-economy figure?

In 2024, the US EPA combined average for new light-duty vehicles is around 26 MPG (US); above 35 MPG is good for gasoline, above 45 MPG is excellent for non-hybrid, and hybrids (Prius, Corolla Hybrid) sit around 50-60 MPG. In L/100km terms, 9 is average, 7 is good, under 5 is excellent. Plug-in hybrids and EVs use different metrics (MPGe or kWh/100km) and cannot be compared directly.

Why does EPA combined differ from manufacturer ratings?

EPA combined is a weighted average of city and highway dynamometer tests (FTP-75, HWFET, US06, SC03, Cold FTP). Manufacturer figures may reflect NEDC or WLTP European cycles, or may quote only highway driving where the number is higher. The EPA publishes its methodology in the Code of Federal Regulations 40 CFR Part 600 and enforces uniform labels in the US. Cross-jurisdiction comparisons are rarely apples-to-apples.

Does the tool handle diesel, gasoline and ethanol-blended fuels differently?

No. The converter is unit-only and does not know what fuel you are using. Diesel carries about 12% more energy per liter than gasoline, so a 6 L/100km diesel uses similar energy to about a 5.4 L/100km gasoline. E85 (85% ethanol) has about 70% of gasoline's volumetric energy, so MPG on E85 drops about 25-30% compared to pure gasoline even though you are paying per gallon. For energy-normalized comparisons, convert fuel volumes to joules using the energy converter.

What about EV efficiency - can this converter handle MPGe?

Not directly. MPG-equivalent (MPGe) is calculated by the EPA using an assumed energy content of 33.7 kWh per US gallon of gasoline. A car rated 100 MPGe consumes 33.7 kWh per 100 miles, or about 210 Wh/km. This tool converts volumetric fuel economy only; for EV efficiency, a kWh/100km or mi/kWh calculator is the right instrument.

Is the Imperial gallon still used?

Yes, in the UK for fuel-economy figures (by tradition), in Canada informally, and in a handful of Caribbean countries. Fuel itself in the UK has been sold in liters since 1995, but MPG on car adverts and road-tax calculations still use Imperial gallons for consumer familiarity. The UK Weights and Measures Act 1985 fixes the imperial gallon at 4.54609 L exactly.

Why are NEDC and WLTP numbers different from real-world MPG?

Dyno tests run standardized drive cycles at controlled temperatures with optimized tire pressures, no accessories, and gentle acceleration. Real drivers use air conditioning, carry passengers, accelerate harder and hit rolling resistance variables the cycle cannot model. Real-world consumption is typically 10-25% worse than the lab figure; the EPA adjusts its label downward to compensate, while NEDC notoriously did not. WLTP (introduced 2017) is closer to reality. This converter works with any number you give it; realism is a separate topic.

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