This guide addresses unit definitions, conversion constants, algebraic derivations, worked examples, and allied quantities that are frequently used by engineers, analysts, fleet managers and interested practitioners who require accurate and reproducible fuel-consumption conversions. The presentation is technical and data-driven, with stepwise calculations and authoritative references for each load-bearing fact.
Units and authoritative constants
- US liquid gallon (US gal). The US liquid gallon is legally defined as exactly 231 cubic inches, equivalent to 3.785411784 litres. This definition is published in NIST Handbook 44 (Appendix C). (NIST)
- Imperial (UK) gallon. The imperial gallon is defined as exactly 4.54609 litres. The imperial gallon is used in a small set of jurisdictions and differs from the US gallon by a factor of approximately 1.20095. (Wikipedia)
- Mile and kilometre. The international statute mile equals 1,609.344 metres (exact). The kilometre is the SI metric unit; therefore the conversion 1 mile = 1.609344 km is exact for the international mile. (Wikipedia)
- Energy content references. The U.S. Energy Information Administration (EIA) reports that one gallon of finished motor gasoline (containing about 10% ethanol by volume) corresponds to 120,214 Btu (an end-use, calendar-year estimate). This figure is used in energy accounting and unit conversions. (EIA)
- Greenhouse gas factor. The U.S. Environmental Protection Agency (EPA) reports the tailpipe carbon dioxide resulting from burning one gallon of gasoline as 8,887 grams CO2 per gallon. The same EPA material explains the rationale for the mass of CO2 exceeding the mass of fuel because oxygen from air is incorporated in combustion products. (EPA)
- EPA energy equivalence for electric vehicles. The EPA expresses the energy content of one US gallon of gasoline as equivalent to 33.7 kilowatt-hours of electricity for the purpose of computing MPGe (miles per gallon gasoline equivalent). The EPA statement reads: “A gallon of gasoline has the energy equivalent of 33.7 kilowatt-hours of electricity.” (EPA)
Basic algebraic conversions: derivation and formulas
Three conversion tasks are encountered repeatedly: (A) volume conversion between gallons and litres, (B) fuel economy conversion between miles per gallon (mpg) and litres per 100 kilometres (L/100 km), and (C) inter-gallon conversions (US ? imperial). Each is presented with definitions, algebraic derivation, and a compact formula for direct use.
A. Gallons ? Litres
Given the exact numerical definition:
- 1 US gallon = 3.785411784 L. (NIST)
Conversion formulae:
- Litres = gallons × 3.785411784.
- Gallons = litres / 3.785411784.
Numeric rounding is left to the user depending on required significant digits.
B. Miles per gallon ? Litres per 100 kilometres
Target representation: litres per 100 kilometres (L/100 km) is the standard fuel-consumption metric in most metric countries. The derivation uses exact definitions above.
Start from mpg (US): mpg = miles / US gallon.
Compute litres per 100 km:
- Convert gallons to litres: 1 US gal = 3.785411784 L.
- Convert miles to kilometres: 1 mi = 1.609344 km.
- The algebraic result (worked) is:
L per 100 km = (100 × 3.785411784) / (1.609344 × mpg) = 235.214583 / mpg.
This simplified constant (approximately 235.214583) is frequently quoted in conversion tools and reference calculators. (Inch Calculator)
Inverse conversion for mpg (US) from a measured L/100 km:
mpg (US) = 235.214583 / (L/100 km)
C. US mpg ? Imperial mpg
Because the imperial gallon is larger, an engine that returns a given mpg in the US system will have a different mpg in the imperial system even if energy use is unchanged.
Conversion factor:
imperial gallon (L) / US gallon (L) = 4.54609 / 3.785411784 ˜ 1.2009499255
Therefore:
- mpg (imp) = mpg (US) × 1.2009499255
- mpg (US) = mpg (imp) ÷ 1.2009499255
The ratio is exact to the available digits when using the defined litre values above. (Wikipedia)
Worked examples (digit-by-digit calculations)
The following worked examples illustrate common conversions and show the arithmetic explicitly.
Example 1 — 30 mpg (US) to L/100 km
Use the constant 235.214583:
L/100 km = 235.214583 / 30 = 7.8404861 L/100 km
Rounded to a typical reporting precision, the value is 7.84 L/100 km.
Example 2 — 8 L/100 km to mpg (US)
Inverse application:
mpg (US) = 235.214583 / 8 = 29.401822875 mpg
Rounded to two decimal places: 29.40 mpg (US).
Example 3 — 50 mpg (US) to imperial mpg
Apply the gallon ratio:
mpg (imp) = 50 × 1.2009499255 = 60.047496275
Rounded: 60.05 mpg (imperial).
Numeric details above are consistent with the standard conversion factors described in the preceding section and with commonly used conversion tools. (Inch Calculator)
Energy content and greenhouse-gas implications
Fuel-consumption conversions are often used as inputs to energy and emissions calculations. Two authoritative reference points are the EIA energy content and the EPA CO2 factor.
Energy content per gallon and per litre
The EIA uses an end-use average value reporting 1 US gallon of finished motor gasoline ˜ 120,214 Btu (this figure accounts for typical fuel formulation including ethanol content). Converting that Btu value into SI units yields the energy per litre.
Stepwise conversion (constants used: 1 Btu = 1,055.05585 J; 1 kWh = 3.6 MJ; 1 US gal = 3.785411784 L):
- Convert Btu to megajoules per gallon: 120,214 Btu/gal × 0.00105505585 MJ/Btu = 126.805 MJ/gal
- Convert per gallon to per litre: 126.805 MJ/gal / 3.785411784 L/gal = 33.5056 MJ/L
- Convert megajoules per litre to kWh per litre: 33.5056 MJ/L / 3.6 MJ/kWh = 9.30711 kWh/L
The derived numbers are consistent with EIA’s tabulated energy content and are presented here for direct use in energy accounting workflows. (EIA)
Tailpipe carbon dioxide per volume of fuel
The EPA provides a single-number factor for tailpipe CO2 from combustion of finished motor gasoline: 8,887 grams CO2 per US gallon. Converting that to a per-litre basis:
8,887 g CO2/gal / 3.785411784 L/gal = 2,347.69703 g CO2/L
Equivalent reporting: ˜2.348 kg CO2 per litre of gasoline burned. This is a direct tailpipe value; life-cycle upstream emissions (extraction, refining, transport) are additional and must be included separately when a well-to-wheel or cradle-to-grave assessment is required. (EPA)
“A gallon of gasoline has the energy equivalent of 33.7 kilowatt-hours of electricity.” (EPA)
“CO2 emissions from a gallon of gasoline: 8,887 grams CO2/gallon.” (EPA)
Both sentences are reproduced exactly from the cited EPA pages and are suitable for inclusion in technical reports that require authoritative phrasing.
Practical quick-reference conversions
Use the following compact table for routine conversions. Each numeric entry is rounded to three significant digits where appropriate; the formulas above should be used for higher precision.
- Volume
- Fuel economy
- L/100 km = 235.214583 / mpg (US). (Inch Calculator)
- mpg (US) = 235.214583 / (L/100 km). (Inch Calculator)
- mpg (imp) ˜ 1.20095 × mpg (US). (Wikipedia)
- Energy & emissions
Use cases and common pitfalls
- Labeling inconsistencies. Dealers, owners and online databases sometimes report fuel economy in imperial mpg or US mpg without explicit units. For cross-border comparisons, confirm which gallon definition is used and then apply the imperial?US conversion factor. (Wikipedia)
- Ethanol blends and energy content. Finished motor gasoline in many markets contains ethanol (E10 is common). The EIA Btu value above reflects typical formulations; modifying the ethanol fraction changes energy density per litre in a predictable way and will change L/100 km ? mpg equivalence on the margin. See EIA documentation for the formulation used in a given data year. (EIA)
- MPGe vs mpg. MPGe (miles per gallon gasoline equivalent) is an EPA construct for energy-equivalent comparison of electric and conventional vehicles. The EPA’s conversion uses 33.7 kWh per gallon as the energy equivalent; MPGe values reported on window stickers include the EPA’s test protocol adjustments (charging losses, etc.). Use MPGe only for energy equivalence, not for direct cost comparisons without additional price data. (EPA)
- Rounding and significant figures. The constants used in the algebraic derivations are exact by legal definition (gallon) or by international agreement (mile). Maintain appropriate significant digits in calculations that feed decision models. Small rounding errors can propagate in fleet-scale calculations; document rounding rules.
Quick methodology checklist for analysts
- Record the unit system of the input data (US gal, imp gal, L/100 km, mpg US, mpg imp).
- Convert volumes first (gallons ? litres) when the data source reports fuel used. (NIST)
- When converting fuel economy, use the derived factor 235.214583 for mpg(US) ? L/100 km. (Inch Calculator)
- For energy or emissions, apply EIA/EPA reference factors and document the source year and whether the gasoline figure includes ethanol content. (EIA)
- Keep a traceable record of every constant and its source in the computation log.
Final Considerations
Fuel-consumption conversion is a routine computational task that benefits from precise definitions, transparent algebra and authoritative source citations. The most frequently used constants are the US gallon (3.785411784 L), the imperial gallon (4.54609 L), the mile (1.609344 km), the L/100 km ? mpg(US) conversion factor (235.214583), the EIA energy content for finished gasoline (120,214 Btu/US gal) and the EPA tailpipe CO2 factor (8,887 g CO2/gal). These sources and numbers are the recommended baseline for reporting, energy accounting and emissions estimation. (NIST) (EIA) (Wikipedia)
For programmatic use or reporting, the guide above provides both the algebraic derivation and concrete numeric examples so that conversions can be implemented in spreadsheets, scripts or reporting pipelines with documented provenance. Where regulatory or accounting precision is required, refer to the primary documents cited in this guide and retain the original wording and reference dates in any formal submission. (EIA)
References (selected)
- NIST Handbook 44, Appendix C — General Tables of Units of Measurement. (NIST)
- U.S. Environmental Protection Agency — Fuel Economy and EV Range Testing. (EPA)
- U.S. Environmental Protection Agency — Greenhouse Gas Emissions from a Typical Passenger Vehicle (CO2 per gallon). (EPA)
- U.S. Energy Information Administration — Energy units and calculators; Motor gasoline energy content (Btu/gal). (EIA)
- Miles-per-gallon conversion references (practical calculators and derivation): omnicalculator / inchcalculator conversion notes. (Omni Calculator)
If a practitioner requires a downloadable worksheet that implements the formulae above and performs batch conversions for an input list of vehicles or trips (including per-litre CO2 outputs and energy totals in MJ or kWh), a ready-to-use spreadsheet can be prepared with embedded citations and documented rounding rules.
