Fuel Economy Converter
Convert MPG, L/100km and km/L correctly — these are reciprocals, not linear factors, and US and Imperial gallons differ by twenty per cent.
Fuel Economy Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Official economy figures come from standardised test cycles and routinely overstate real-world results.
- Driving style, load, terrain, temperature and tyre pressure affect consumption more than most unit differences.
- MPGe compares energy at the vehicle and ignores generation and transmission losses entirely.
- A bare mpg figure is ambiguous by about twenty per cent unless the gallon is identified.
In short: Fuel figures come in two opposite kinds. Economy — mpg, km/L — is distance per fuel, where higher is better. Consumption — L/100km — is fuel per distance, where lower is better. They are reciprocals, so you divide a constant rather than multiply: L/100km = 235.2146 ÷ US mpg, or 282.4809 for Imperial. That reciprocal is also why improving 10 → 15 mpg saves more fuel than 30 → 60.
Formula
Economy units measure distance per unit of fuel and consumption units measure fuel per unit of distance, so the two groups are reciprocals. The constant 235.2146 is one hundred times the litres in a US gallon divided by the kilometres in a mile; the Imperial constant is larger purely because the Imperial gallon is larger.
Worked Example
- Identify the kind. Miles per gallon is an economy unit; litres per 100 km is a consumption unit.
- Recognise the inversion. Crossing between them means dividing, not multiplying.
- Apply the constant. 235.2146 ÷ 30 = 7.840486 L/100km.
- Check the gallon. The same 30 mpg on the Imperial scale would be 9.416031 L/100km instead.
- Sense-check the direction. Higher mpg must always give a lower L/100km.
The illusion worth understanding. Replacing a 10 mpg vehicle with a 15 mpg one saves 7.8405 litres per 100 km. Replacing a 30 mpg car with a 60 mpg hybrid — doubling the economy — saves only 3.9202. The five-mpg improvement at the bottom of the range saves twice as much fuel as the thirty-mpg improvement at the top. Miles per gallon hides this completely because it is a reciprocal scale; litres per 100 km makes it obvious at a glance, which is the strongest argument for the metric convention.
Strengths & Limits Of This Model
Where this engine is strong
- Handles the economy-consumption inversion correctly instead of applying a linear factor.
- Always reports both US and Imperial mpg, never one alone.
- Quantifies the mpg illusion so unequal savings are visible.
- Constants derived from the exact gallon and mile definitions.
Where it stops
- Cannot predict your actual consumption, which depends on how and where you drive.
- Does not model fuel cost, which needs a price and a distance as well.
Practical Use Cases
Comparing vehicles across markets
American window stickers quote US mpg, British reviews quote Imperial mpg and European data quotes L/100km. The same car produces three very different numbers, and only conversion makes them comparable.
Fleet fuel budgeting
Consumption units are the right basis for budgeting because fuel cost scales linearly with litres per distance, not with mpg. Ranking vehicles by mpg systematically overstates the benefit of upgrading already-efficient ones.
Deciding which vehicle to replace first
The reciprocal relationship means the worst vehicle in a fleet is almost always the highest-value replacement, even for a modest mpg gain.
Reading imported specifications
Japanese and Indian figures often appear in km/L while the same model is sold elsewhere in L/100km — pair with the Volume Converter when the tank capacity is quoted in the other system too.
Methodology & Editorial Standards
This engine does not use the linear factor model that the other converters on this site use, because fuel figures are not all linear in one another. Units are classified as economy, meaning distance per unit of fuel, or consumption, meaning fuel per unit of distance. Everything is normalised to kilometres per litre and then converted out, so crossing between the two kinds correctly inverts rather than scaling. The constants follow from exact definitions: a US gallon is 231 cubic inches, or 3.785411784 litres, and a mile is exactly 1.609344 km, which gives 235.2145833 for the US mpg relationship; the Imperial gallon of exactly 4.54609 litres gives 282.4809364. Both are shown simultaneously because a bare mpg figure is ambiguous by about twenty per cent between the two markets. The comparison readout exists to make the reciprocal behaviour concrete, since equal steps in mpg represent very unequal changes in actual fuel use. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Fuel Economy Converter — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I convert MPG to L/100km?
Divide 235.2146 by the US mpg figure. So 30 mpg is 235.2146 ÷ 30 = 7.840486 L/100km. If the figure is Imperial mpg, divide 282.4809 instead. It is a division rather than a multiplication because the two units are reciprocals: one measures distance per fuel and the other fuel per distance.
Why is US MPG different from UK MPG?
Because the gallons differ. A US gallon is 3.785412 litres; an Imperial gallon is 4.54609, about 20.09 per cent larger. The same car therefore scores about 1.2009 times higher on the Imperial scale. A car rated 30 mpg in America is 36.03 mpg in Britain, with no change to the vehicle whatsoever.
Where does the number 235.2146 come from?
From the exact definitions. One US gallon is 3.785411784 litres and one mile is 1.609344 kilometres. Multiplying the litres per gallon by 100 and dividing by the kilometres per mile gives 235.2145833. The Imperial constant of 282.4809 is derived identically from the larger gallon.
Why is the conversion not a simple multiplication?
Because economy and consumption run in opposite directions. Miles per gallon rewards a bigger number; litres per 100 km rewards a smaller one. Converting between a quantity and its reciprocal is always a division into a constant. Applying a fixed multiplier would give the right answer at exactly one point and be wrong everywhere else.
Does improving from 30 to 35 MPG save as much as 15 to 20?
Not remotely. Going from 15 to 20 mpg saves 3.9202 litres per 100 km, while 30 to 35 saves only 1.1201 — less than a third as much, for the same five-mpg improvement. This is the mpg illusion, and it is why fuel policy analysts prefer consumption units.
Which is better, higher or lower?
Higher is better for mpg and km/L, because they measure how far you travel on a given amount of fuel. Lower is better for L/100km and gallons per 100 miles, because they measure how much fuel a fixed journey consumes. Getting this backwards is the most common error when reading foreign specifications.
What is km/L and who uses it?
Kilometres per litre is an economy unit common in Japan, India, South Korea and parts of Latin America. It is metric but runs the same way round as mpg, higher being better. To convert to L/100km, divide 100 by the km/L figure.
What is MPGe for electric vehicles?
Miles per gallon equivalent, an EPA construct that treats 33.7 kilowatt hours as energetically equal to one US gallon of petrol. It allows a rough comparison with combustion vehicles but compares energy at the vehicle, not at the source, so it says nothing about how the electricity was generated.
Do real-world figures match the official ones?
Usually not. Official figures come from standardised test cycles run under controlled conditions, and real driving involves cold starts, traffic, terrain, load, tyre pressure and speed. Shortfalls of ten to twenty per cent against the sticker are common, and the gap tends to be larger for hybrids in cold weather.
How do I compare two cars fairly?
Convert both to the same unit first, and prefer a consumption unit such as litres per 100 km for the comparison. Because fuel cost scales linearly with consumption rather than with mpg, the consumption figure is what maps directly onto money spent.