Fuel Economy Converter

Introduction

A fuel-economy figure is one of the most frequently converted numbers in everyday life, but it sits at the intersection of three different unit systems that disagree on the size of a gallon, the length of a mile, and whether consumption is "per 100 km" or "per gallon". A US-market window sticker says MPG (US); a European brochure says L/100km; a Japanese spec sheet says km/L; a UK owner manual may say MPG (UK / imperial). The numbers are not the same, the conversion factors are not round, and a single typo on the wrong gallon can swing the result by 20%. This Fuel Economy Converter exists to remove that whole class of mistake. It accepts any of the nine most common road-vehicle units, converts it through a single canonical scalar (litres per 100 kilometres, the same scalar the US EPA, the EU WLTP cycle, ISO 4038, and SAE J1342 use for regulatory reporting), and shows the equivalent figure in every other unit at the same time.

A second purpose of the tool is to anchor the conversion in reality. The live results table sits next to a "Reference vehicle efficiency" panel that lists the approximate combined-cycle fuel economy of seven typical vehicles, from a Toyota Corolla Hybrid at the efficient end, through a Ford F-150 pickup, a Tesla Model 3 EV, a touring motorcycle, and on to a Class-8 semi-truck at the other extreme. The panel is for orientation only: actual fuel economy varies with driving conditions, and every figure in the panel is given as a range rather than a precise number. A cost-per-distance helper below the converter combines the fuel-economy figure with a fuel price to give the running cost per kilometre or per mile, the calculation most drivers actually want when comparing two vehicles.

How to Use

1. Type a number into the Value field at the top of the tool. The default is 30, which corresponds to a typical US-market sedan. Positive numbers only, fuel-economy and fuel-consumption figures are not defined for zero or negative inputs.
2. Pick the From unit. The dropdown lists every unit the converter understands: MPG (US), MPG (UK / Imperial), km/L, L/100km, L/10km, L/km, Miles / Litre, km per US gallon, and km per UK (imperial) gallon. Each row in the live results table has a short blurb explaining what the unit means and where it is most commonly used.
3. Choose the Significant figures selector. The converter offers 4, 6, or 8 significant digits. Four is enough for back-of-envelope comparisons; six is the default for engineering and dealer-lot work; eight covers regulatory submissions and fuel-economy research.
4. Read the live results table. Every other unit appears in its own row, formatted to the chosen significant-figure precision. The input unit is marked with a small "(input)" tag. The third column gives a one-line description of each unit so the table doubles as a quick reference card.
5. Use the preset buttons to load common figures: 30 MPG (US) for a typical US sedan, 50 MPG (US) for a modern hybrid, 7 L/100km for an efficient diesel, 10 L/100km for a small petrol, and 15 km/L for an Asian-market rating.
6. Open the cost-per-distance helper below the results table. Type a distance, pick km or miles, type a fuel price, pick the unit the price is quoted in (per litre, per US gallon, or per UK gallon), and edit the currency symbol if it is not a US dollar. The helper returns total fuel in litres, total cost, cost per distance unit, and the effective price per litre. It uses the fuel-economy figure currently loaded in the converter at the top of the page.
7. Open the reference vehicle panel to see approximate combined-cycle fuel economy for seven typical vehicles, and to load any of those figures into the converter with one click.

The Formulas

All nine units are converted through a single canonical scalar: litres per 100 kilometres (L/100km). This is the regulatory figure used by the US EPA, the EU WLTP test cycle, ISO 4038 (Road vehicles, Fuel consumption measurement), and SAE J1342 (Fuel Economy Test Procedure, Type I). The conversion matrix collapses neatly because the nine units fall into three groups.

Economy units (distance per unit volume of fuel). MPG (US), MPG (UK), km/L, miles per litre, km per US gallon, and km per UK gallon are all "distance per volume" units. Converting any one of them to L/100km first divides the distance into the volume to get km/L (then L/100km = 100 / km/L), then converts the volume axis if the unit is per gallon. The exact factors, defined by NIST and the international yard and pound agreement, are 1 US gallon = 3.785 411 784 L (exact), 1 UK / imperial gallon = 4.546 09 L (exact), 1 mile = 1.609 344 km (exact). The two key derived constants are 235.215 (= 100 × 3.785 411 784 / 1.609 344) and 282.481 (= 100 × 4.546 09 / 1.609 344), which convert between MPG and L/100km in either direction: L/100km = 235.215 / MPG(US) and L/100km = 282.481 / MPG(UK).

Consumption units (volume per unit distance). L/100km, L/10km, and L/km are the consumption side of the same family. They differ only by a factor of 10 between adjacent forms (L/10km = L/100km / 10; L/km = L/100km / 100). Because they are already in the canonical scalar, no further conversion is needed.

Per-gallon volume axis. The two "km per gallon" units mix the km/L economy form with a non-metric gallon volume: km/gal(US) = (100 / L/100km) × 3.785 411 784, and km/gal(UK) = (100 / L/100km) × 4.546 09. The UK factor is exactly 1.200 95 times the US factor, the same ratio that distinguishes the two gallons themselves.

Cost per distance. The cost panel multiplies the volume of fuel required for the chosen distance by the fuel price. The volume is L/100km × distance / 100 if the distance is in km, and L/km × distance if the distance is in miles (with L/km = L/100km / 100). The price is converted to a per-litre basis by dividing by 3.785 411 784 (US gal) or 4.546 09 (UK gal) if the user has entered a per-gallon price. Total cost, cost per distance unit, total litres, and effective price per litre are then returned. All numbers round-trip exactly within floating-point precision, because every factor in the chain is a ratio of exact constants.

Worked Examples

30 MPG (US) to L/100km. 30 miles per US gallon equals 30 × 1.609 344 = 48.280 32 km per 3.785 411 784 L, or 12.754 3 km/L. The reciprocal of 12.754 3 km/L scaled to 100 km is 100 / 12.754 3 = 7.840 5 L/100km. This is the canonical textbook example and the most-memorised single conversion in the field. The same number via the shortcut formula 235.215 / 30 = 7.840 5 L/100km.

10 L/100km to MPG (US). 10 litres per 100 km is 0.1 L per 1 km, or 10 km per 1 L. Ten km/L is 10 / 1.609 344 = 6.213 71 miles per litre. Multiplying by 3.785 411 784 L/US gal gives 6.213 71 × 3.785 411 784 = 23.521 5 MPG (US). The shortcut 235.215 / 10 = 23.521 5 matches.

50 MPG (US) to MPG (UK). A vehicle that gets 50 MPG on the smaller US gallon is also doing 50 × 3.785 411 784 / 4.546 09 = 41.633 7 MPG on the larger UK gallon. Equivalently, L/100km = 235.215 / 50 = 4.704 3, then MPG (UK) = 282.481 / 4.704 3 = 60.047 5. The two routes agree.

7 L/100km to km/L and to MPG (US). 7 L per 100 km is 100 / 7 = 14.285 7 km/L. Converting to MPG (US): 235.215 / 7 = 33.602 1 MPG (US). This corresponds to a moderately efficient European diesel.

15 km/L to L/100km and to MPG (UK). 15 km per litre is 100 / 15 = 6.666 7 L/100km. Converting to MPG (UK) via the L/100km form: 282.481 / 6.666 7 = 42.372 1 MPG (UK). Converting via the direct form: 15 km/L = 15 / 1.609 344 = 9.320 6 mi/L, then 9.320 6 × 4.546 09 = 42.371 4 MPG (UK). The two routes agree to four significant figures.

Cost per 100 km at 10 L/100km and $1.45 per litre. Volume = 10 L. Cost = 10 × 1.45 = $14.50. Cost per km = $0.145. Cost per mile = $0.145 × 1.609 344 = $0.233 4. If the price is quoted per US gallon instead ($5.50/US gal, the 2024 US average), the effective per-litre price is $5.50 / 3.785 411 784 = $1.452 9, and the same trip cost is essentially unchanged: 10 × 1.452 9 = $14.53.

Where It Shows Up

Vehicle window stickers and owner manuals. In the US, every new passenger car and light truck carries an EPA fuel-economy label listing MPG (US) for city, highway, and combined driving. In the EU, the WLTP certificate lists L/100km and (since 2020) a WLTP-to-MPG conversion. In the UK, an older manual or used-car listing may quote MPG (UK). A buyer cross-referencing a US-spec car against a UK-spec one, or a vintage car against a modern one, needs exactly the conversion this tool provides.

Regulatory submissions and fleet management. Fleet operators report fuel consumption in L/100km to the US DOE AFDC and the EU EEA, in MPG (US) to the US EPA, and sometimes in L/km or kg CO₂/km for the most recent regulatory year. The cross-conversion is needed any time a fleet spans more than one reporting jurisdiction. Class-8 trucking fleets in particular report in both mpg (US) and L/100km to satisfy both DOT and EPA requirements.

Owner-manual translation and second-hand imports. A car imported from Japan into Europe typically carries a km/L figure in its original spec sheet and an L/100km figure on the EU paperwork. The two are exact reciprocals of each other scaled by 100, but the document often uses the older "10·15 mode" or JC08 cycle rather than the more recent WLTP cycle, which means the figure may not match the European label exactly. This converter handles the unit axis but not the test-cycle difference, which is the bigger real-world source of confusion.

Fuel-economy research and journalism. Academic and policy papers compare fuel economy across countries and time periods. The most common source of error is mixing MPG (US) and MPG (UK), and the second is mixing L/100km (consumption) with MPG (economy) without flipping the inversion. Both mistakes are easy to catch with a quick cross-check through a single converter.

Race-engineering, e-bikes, and small engines. Motorsport teams report fuel usage in L/100km-equivalent figures derived from lap-time and fuel-flow data, in L/lap for circuit races, and in mpg (US) for endurance events. L/10km is a useful unit for short-distance small engines because the absolute consumption is small. Marine and aviation engines are usually rated in L/km or L/nautical mile, because the absolute consumption is much higher than for a car. The converter handles L/km as a first-class unit so the comparison is one click.

Common Mistakes

Mixing MPG (US) and MPG (UK). The single most common fuel-economy mistake in cross-border comparisons. A US-spec car rated 30 MPG (US) is genuinely more efficient than a UK-spec car rated 30 MPG (UK), because the US gallon is about 17% smaller. The correct equivalent of 30 MPG (US) is about 36 MPG (UK), and the correct equivalent of 30 MPG (UK) is about 25 MPG (US). Cross-references that don't apply the 1.200 95 ratio silently overstate or understate efficiency by 20%.

Mixing consumption and economy units without flipping the inversion. L/100km is a consumption figure (volume per distance); MPG is an economy figure (distance per volume). They are reciprocals of each other scaled by 235.215. A naive scaling like "30 MPG is about 8 L/100km, so 50 MPG is about 13 L/100km" is wrong: the higher the MPG, the lower the L/100km. The correct relationship is L/100km = 235.215 / MPG.

Forgetting that L/100km is a regulatory cycle figure, not a real-world figure. Every published L/100km or MPG figure on a new-car window sticker is the result of a laboratory test cycle (EPA FTP-75 in the US, WLTP in the EU, JC08 in Japan). Real-world fuel economy is typically 10 to 25% lower than the test-cycle figure, with the gap depending heavily on driving style, traffic, climate, and load. The reference vehicle panel on this page deliberately presents all figures as ranges and explicitly notes the real-world variation, because single-point figures are misleading.

Using the wrong gallon for a per-gallon price. Fuel prices are sometimes quoted per US gallon (US and Canada), sometimes per litre (most of the world), and sometimes per UK gallon (some Commonwealth markets and a few historical pricing references). Mixing the volume units is an easy way to mis-estimate running cost by 20%. The cost-per-distance helper on this page lets the user pick the unit the price is quoted in and converts it to a per-litre basis internally.

Off-by-10 errors between L/100km, L/10km, and L/km. The three consumption units differ by a factor of 10 between adjacent forms and 100 between L/100km and L/km. A spec sheet that reads "0.78 L/km" is the same car as one that reads "7.8 L/100km", not "78 L/100km". The converter handles all three as separate units precisely so the user can copy the figure directly from the source without doing the 10× shift in their head.

Treating MPGe as MPG. MPGe (miles per gallon equivalent) is an EPA energy-equivalence figure used for battery-electric and plug-in hybrid vehicles. 33.7 kWh of electricity is defined as equivalent to one US gallon of gasoline for MPGe purposes. A Tesla Model 3 Long Range rated at 130 MPGe is not directly comparable to a Toyota Corolla Hybrid rated at 50 MPG on a fuel-volume basis, because the units measure different things (energy vs. volume). The reference panel on this page lists MPGe figures for the Model 3 and notes the difference explicitly.

Frequently Asked Questions

What is the difference between MPG (US) and MPG (UK)?

A US gallon is exactly 3.785 411 784 litres; an imperial (UK) gallon is exactly 4.546 09 litres, or about 20% larger. The same vehicle therefore shows a smaller MPG number on a US gallon basis than on a UK gallon basis. 50 MPG (US) corresponds to about 41.6 MPG (UK), and 50 MPG (UK) corresponds to about 60.0 MPG (US). This is the single most common source of confusion in cross-border owner-manual comparisons and in second-hand vehicle imports.

How do I convert MPG (US) to L/100 km?

Divide 235.215 by the MPG (US) figure. The 235.215 factor is 100 × 3.785 411 784 / 1.609 344, the volume of 100 US gallons expressed in litres, divided by the distance of 100 miles expressed in kilometres. For example, 30 MPG (US) = 235.215 / 30 ≈ 7.84 L/100km, the canonical textbook example. The reverse direction is MPG (US) = 235.215 / (L/100km). For MPG (UK), use the analogous constant 282.481 (= 100 × 4.546 09 / 1.609 344).

Why does the L/100km figure go down as the car gets more efficient?

L/100km is a consumption figure (volume per distance), while MPG is an economy figure (distance per volume). The two are reciprocals of each other: a car that uses less fuel to cover 100 km returns a smaller L/100km number and a larger MPG number. The relationship is L/100km = 235.215 / MPG(US) and MPG(US) = 235.215 / L/100km. So a more efficient car has a lower L/100km and a higher MPG, and the change runs in opposite directions for the two scales. This is the most common conceptual stumbling block in the field, because everyday English associates "more" with "better" while the consumption scale inverts that.

What does MPGe mean and is it the same as MPG?

MPGe (miles per gallon equivalent) is a US EPA figure for battery-electric and plug-in hybrid vehicles. It expresses electrical energy in the same units as a gallon of gasoline: 33.7 kWh of electricity is defined as the energy equivalent of one US gallon of gasoline. A Tesla Model 3 Long Range is rated at approximately 120 to 135 MPGe combined, depending on the model year and wheel size. MPGe is not the same as MPG because it measures energy, not fuel volume, and the underlying unit conversions are different. A direct comparison between a 130 MPGe EV and a 50 MPG hybrid on a fuel-volume basis is not meaningful, the right comparison is on energy content or on cost per mile.

What fuel-economy test cycles are used around the world?

Three major regulatory cycles dominate. The US EPA test cycle (FTP-75 for city driving plus US06 and SC03 for high-speed and air-conditioning load) is the basis for the window-sticker MPG figure. The EU WLTP (Worldwide harmonised Light vehicles Test Procedure) plus the older NEDC is the basis for European L/100km figures. The Japanese JC08 cycle was the basis for Japanese km/L figures until WLTP adoption. SAE J1342 and ISO 4038 specify the laboratory test procedure itself. Real-world fuel economy is typically 10 to 25% lower than the test-cycle figure, with the gap depending on driving style, traffic, climate, and load.

Can the Fuel Economy Converter be used for professional or commercial purposes?

Yes, the calculator provides mathematically correct results that are suitable for professional, commercial, and educational use. For the Fuel Economy Converter, For the Fuel Economy Converter, For high-stakes applications (medical, legal, financial), verify results with a domain expert. For the Fuel Economy Converter, the Fuel Economy Converter formulas used are well-established and validated against reference standards.

For the Fuel Economy Converter, How often are the underlying formulas updated?

For the Fuel Economy Converter, the Fuel Economy Converter formulas are based on established scientific, mathematical, or industry-standard references and rarely require updates. When standards change (e.g., new physical constants, revised tax brackets, updated standards), the Fuel Economy Converter is updated to reflect the current authoritative source. For the Fuel Economy Converter, Each calculator's references section lists the specific sources used.

References

• US Environmental Protection Agency, Fuel Economy Labeling of Motor Vehicles, Revisions to Improve Calculation of Fuel Economy Estimates (40 CFR Part 600). The regulatory basis for the US window-sticker MPG figure. Publicly available at https://www.ecfr.gov/current/title-40/chapter-I/subchapter-Q/part-600 and via the EPA FuelEconomy.gov portal (https://www.fueleconomy.gov/).
• US Department of Energy, Alternative Fuels Data Center (AFDC), Vehicle Cost Calculator and Fleet Average Data. The reference source for US fleet-average fuel-economy data, including Class-8 trucking fleet averages, and for the energy-equivalence constant 33.7 kWh = 1 US gallon gasoline that defines MPGe. https://afdc.energy.gov/.
• SAE International, SAE J1342, Fuel Economy Test Procedure (Type I) for Automotive Vehicles. The laboratory test procedure used by most US OEM fuel-economy submissions.
• International Organization for Standardization, ISO 4038:1996, Road vehicles, Fuel consumption measurement. Specifies the consumption measurement procedure referenced by EU and international type-approval.
• National Institute of Standards and Technology, NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. Authoritative source for the exact definition of the US liquid gallon (3.785 411 784 L) and the US relationship to the international yard and pound (1 mile = 1.609 344 km). https://www.nist.gov/pml/weights-and-measures/publications/nist-handbooks/handbook-44.