Fuel Economy Converter
Convert between miles per gallon and liters per 100 kilometers — the two dominant, inversely-related ways fuel efficiency is expressed around the world.
Inputs
- Value
- Convert
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Saved Scenarios
— select 2+ to compare| Metric | |
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Result
7.84
Spark says
How it's calculated
Formula
- MPG
- — Miles per US gallon
- L/100km
- — Liters consumed per 100 kilometers
What is the Fuel Economy Converter?
MPG (miles per gallon) and L/100km (liters per 100 kilometers) measure fuel economy inversely — higher MPG is more efficient, while lower L/100km is more efficient. This calculator converts between the two.
Use this when comparing a vehicle's fuel economy rating across US and international specifications, researching a car for import or export where the fuel economy figure is quoted in an unfamiliar convention, or estimating fuel costs for a trip using a rating from a different measurement system.
How to use it
- 1 Enter the value.
- 2 Choose the conversion direction.
Understanding Fuel Economy Converter
Fuel economy measurement splits along a genuinely interesting philosophical line: the United States (and a few other countries) express it as distance traveled per unit of fuel (miles per gallon), while most of the rest of the world expresses it as fuel consumed per unit of distance (liters per 100 kilometers) — and these aren't just different units for the same underlying concept, they're inverted framings of it, which is exactly why converting between them isn't a simple multiplication by a constant factor, unlike most other unit conversions.
MPG asks 'how far can I go on a fixed amount of fuel,' framing efficiency as a distance-maximization question — a higher number is always better, which maps intuitively onto a driver's mental model of 'getting more miles out of a tank.' L/100km asks the inverse question: 'how much fuel do I need for a fixed distance,' framing efficiency as a consumption-minimization question — here, a lower number is better, which maps more directly onto actual fuel cost and consumption planning, since it's genuinely answering 'how much will this trip cost me in fuel' more directly than MPG does.
Because these two units measure genuinely reciprocal relationships (distance-per-fuel versus fuel-per-distance) rather than simply different-sized versions of the same measurement, the conversion between them involves a division, not a multiplication — which is also why, mathematically, the exact same formula works in both directions: dividing the same constant (which bundles together the gallon-to-liter and mile-to-kilometer conversion factors) by the input value produces the output in either direction. This reciprocal relationship also explains a genuinely counterintuitive property worth understanding: equal-sized improvements in MPG don't correspond to equal-sized improvements in L/100km, especially at the high-efficiency end of the scale. Improving from 10 MPG to 20 MPG saves dramatically more fuel over a fixed distance than improving from 40 MPG to 50 MPG, even though both represent the same '10 MPG improvement' — this is because the relationship is reciprocal, not linear, and it's a large part of why fuel-economy policy discussions increasingly favor gallons-per-mile or L/100km framing over raw MPG for evaluating the real-world fuel savings of efficiency improvements, since the consumption-based framing more accurately reflects actual fuel saved.
A further wrinkle worth knowing when comparing figures internationally: 'gallon' itself isn't a single universal unit. The US gallon (3.785 liters) and the UK imperial gallon (4.546 liters) are meaningfully different sizes, a historical divergence from when the two countries standardized their measurement systems separately. A UK-sourced MPG figure, using the larger imperial gallon, isn't directly comparable to a US MPG figure without first accounting for this gallon-size difference — the same underlying vehicle efficiency, expressed in UK MPG versus US MPG, will show as different numbers purely due to this unit-size mismatch, independent of any real difference in the vehicle's actual fuel efficiency.
Worked examples
Advantages
- •Handles the inverse relationship between MPG and L/100km correctly, avoiding a common manual conversion mistake.
- •Makes vehicle fuel economy directly comparable across US and most international specifications.
- •Useful for both casual comparison shopping and more precise fuel cost or trip planning.
- •Single shared formula in both directions, reflecting the genuine reciprocal relationship between the two units.
Limitations
- •Uses the US gallon (3.785 L) — UK/imperial gallons differ, so a UK-sourced MPG figure needs an additional adjustment before this conversion applies directly.
Common mistakes
- ⚠️ Treating MPG and L/100km as if they scale the same direction — MPG is 'higher is better' (more distance per unit fuel) while L/100km is 'lower is better' (less fuel per unit distance), an inverse relationship that trips up quick comparisons.
- ⚠️ Assuming a UK MPG figure uses the same gallon size as a US MPG figure — the imperial gallon is meaningfully larger than the US gallon, so UK MPG figures aren't directly comparable to US MPG figures without an additional conversion.
- ⚠️ Averaging MPG figures directly when combining trip segments of different distances, when a more accurate blended average requires accounting for fuel actually used, not simply averaging the two MPG numbers.
Tips
- 💡 Remember the direction: MPG is better when higher, L/100km is better when lower — the two scales run in opposite directions, so a quick glance can mislead if you're used to one system and reading the other.
- 💡 If comparing a UK-sourced MPG figure, confirm whether it uses the imperial gallon (about 4.546 L) rather than the US gallon (3.785 L) this calculator uses, since the two produce different results.
- 💡 For blending fuel economy across a multi-segment trip with different distances, calculate total fuel used and total distance separately, then divide, rather than simply averaging two MPG figures.
- 💡 Use L/100km for more intuitive fuel cost estimation in metric contexts, since it directly represents fuel needed for a fixed, meaningful distance.
Real-life uses
- Comparing a vehicle's fuel economy rating across US and international specifications
- Researching an imported or exported car whose fuel economy is quoted in an unfamiliar system
- Estimating fuel costs for a trip using a rating from a different measurement convention
- Understanding a car review or spec sheet that mixes MPG and L/100km figures
Frequently asked questions
Why is the conversion the same formula both ways?
Because MPG and L/100km are reciprocally related through the same constant (235.214583) — dividing that constant by either value gives the other.
Why doesn't a 10 MPG improvement always save the same amount of fuel?
Because MPG and fuel consumption are reciprocally, not linearly, related — going from 10 to 20 MPG saves far more fuel over a fixed distance than going from 40 to 50 MPG, even though both are a '10 MPG' improvement.
Is a UK gallon the same as a US gallon?
No — the UK imperial gallon (about 4.546 L) is meaningfully larger than the US gallon (3.785 L) this calculator uses, so UK MPG figures need an additional adjustment before comparing directly to US MPG figures.
Why does the rest of the world use L/100km instead of MPG?
L/100km directly answers 'how much fuel will this trip cost me,' a consumption-focused framing, while MPG answers 'how far can I go on this tank,' a distance-focused framing — most countries outside the US adopted the consumption-based convention.
Should I average MPG figures when combining trip segments?
Not directly — a more accurate blended fuel economy figure comes from totaling actual fuel used and actual distance covered across all segments, then dividing, rather than simply averaging the individual MPG numbers.
Sources & references
calixo.cloud/unit-conversion/fuel-economy-converter/ — free calculator, no signup required.