Why can't I just multiply MPG by a constant to get L/100km?

Because they measure opposite things. MPG is distance-per-fuel (bigger is better); L/100km is fuel-per-distance (smaller is better). Converting between a rate and its inverse requires dividing, not multiplying by a fixed factor — which is why simple unit-conversion tables that just multiply MPG by a constant produce wrong results.

What's the actual formula?

For US gallons and kilometers: L/100km = 235.214 / MPG (US). The 235.214 constant combines the gallon-to-liter conversion (3.78541 L per US gallon) with the mile-to-kilometer conversion (1.60934 km per mile), scaled to a 100 km basis. For UK (imperial) gallons, the constant is 282.481 instead, since an imperial gallon is larger than a US gallon.

Does going from 40 to 20 MPG double fuel use?

Yes, roughly — but going from 40 to 20 MPG is a much bigger fuel-cost jump than going from 15 to 10 MPG, even though both are a 20 MPG-equivalent-feeling drop in a naive linear reading. Because the relationship is inverse, the same MPG improvement matters far more at the low end (poor fuel economy) than at the high end — improving from 10 to 15 MPG saves far more fuel over the same distance than improving from 40 to 50 MPG.

Why do US and UK MPG differ for the same car?

A US gallon (3.785 liters) is smaller than an imperial/UK gallon (4.546 liters). The same physical fuel efficiency yields a higher MPG number when measured in the larger UK gallon, since each gallon carries the car further in gallon-count terms. A car rated 30 MPG (US) is about 36 MPG (UK) at the same real-world efficiency — no actual difference in the car, just the size of the gallon.

Guide

MPG to L/100km: Why It Isn't Linear

MPG asks how far one gallon goes; L/100km asks how much fuel one hundred kilometers costs — flipped questions that don't scale together linearly.

By Buğra SözeriPublished September 25, 2026

A car spec sheet listing “30 MPG” in one market and “7.8 L/100km” in another isn’t giving you two independent facts — it’s the same fuel efficiency phrased as two structurally opposite questions. Converting between them correctly means dividing, not multiplying by a flat constant, and the fuel economy converter handles that inversion automatically across MPG, L/100km, and km/L.

Two opposite kinds of question

MPG (miles per gallon) and km/L (kilometers per liter) are both distance-per-fuel— they answer “how far does one unit of fuel take me?” A bigger number means better efficiency.

L/100km flips the question to fuel-per-distance— “how much fuel does a fixed distance cost?” A smaller number means better efficiency. Because one side of the conversion is “more is better” and the other is “less is better,” the relationship between the two numbers is inverse, not linear — you can’t convert one to the other with a single multiplication the way you can convert, say, miles to kilometers.

The formula

L/100km = 235.214 ÷ MPG (US)

The 235.214 constant folds together the US gallon-to-liter factor (3.78541 liters per gallon) and the mile-to-kilometer factor (1.60934 km per mile), rescaled so the result lands on a “per 100 km” basis. For UK/imperial gallons, which are larger than US gallons, the constant changes to 282.481 instead, since more liters fit in an imperial gallon.

Worked example: a car rated 30 MPG (US). 235.214 ÷ 30 ≈ 7.84 L/100km. Halve the MPG to 15, and L/100km doesn’t simply double to roughly 15.7 by coincidence — it does in this specific case because dividing a constant by half the denominator exactly doubles the result, which is a property of inverse relationships in general: halving one side always doubles the other, regardless of where you start on the scale.

Why the same MPG gap means very different things

Because the relationship is inverse, a fixed MPG improvement doesn’t translate to a fixed L/100km improvement — the size of the effect depends heavily on where you start:

MPG change	L/100km change	Fuel saved per 100km
10 → 15	23.5 → 15.7	7.8 L
20 → 25	11.8 → 9.4	2.4 L
40 → 50	5.9 → 4.7	1.2 L

The same 5-MPG improvement saves nearly seven times as much actual fuel when it happens at the low end of the scale (10 to 15 MPG) as it does at the high end (40 to 50 MPG). This is a real, well-documented effect of the inverse relationship — not a rounding artifact — and it’s why fuel-economy policy analysts generally argue that pushing the least efficient vehicles on the road up a few MPG matters more for total fuel consumption than the same MPG gain on already efficient vehicles.

US MPG vs UK MPG: a second, unrelated gap

A separate wrinkle: “MPG” alone is ambiguous between US and UK/imperial gallons, which are different sizes. A UK gallon (4.546 liters) holds about 20% more than a US gallon (3.785 liters), so the same car’s real fuel efficiency produces a higher MPG number when quoted in UK gallons than in US gallons — roughly 30 MPG (US) equals about 36 MPG (UK) for an identical vehicle. This isn’t the inverse-relationship issue above; it’s a plain unit conversion, but it compounds with the inversion problem when comparing a US spec sheet in MPG to a European one in L/100km. The fuel economy converter handles both conversions — US/UK gallon and the MPG/L/100km inversion — in one pass. For how EPA sticker numbers compare to real-world driving on top of this, see fuel economy: real-world vs EPA.

Frequently asked questions

Why can't I just multiply MPG by a constant to get L/100km?: Because they measure opposite things. MPG is distance-per-fuel (bigger is better); L/100km is fuel-per-distance (smaller is better). Converting between a rate and its inverse requires dividing, not multiplying by a fixed factor — which is why simple unit-conversion tables that just multiply MPG by a constant produce wrong results.
What's the actual formula?: For US gallons and kilometers: L/100km = 235.214 / MPG (US). The 235.214 constant combines the gallon-to-liter conversion (3.78541 L per US gallon) with the mile-to-kilometer conversion (1.60934 km per mile), scaled to a 100 km basis. For UK (imperial) gallons, the constant is 282.481 instead, since an imperial gallon is larger than a US gallon.
Does going from 40 to 20 MPG double fuel use?: Yes, roughly — but going from 40 to 20 MPG is a much bigger fuel-cost jump than going from 15 to 10 MPG, even though both are a 20 MPG-equivalent-feeling drop in a naive linear reading. Because the relationship is inverse, the same MPG improvement matters far more at the low end (poor fuel economy) than at the high end — improving from 10 to 15 MPG saves far more fuel over the same distance than improving from 40 to 50 MPG.
Why do US and UK MPG differ for the same car?: A US gallon (3.785 liters) is smaller than an imperial/UK gallon (4.546 liters). The same physical fuel efficiency yields a higher MPG number when measured in the larger UK gallon, since each gallon carries the car further in gallon-count terms. A car rated 30 MPG (US) is about 36 MPG (UK) at the same real-world efficiency — no actual difference in the car, just the size of the gallon.

Sources & references

Authoritative references cited by this piece. Verified by Buğra Sözeri on the dates shown and re-checked at every deploy.

US EPA — Fuel Economy conversion factors — Reference for US gallon, mile-to-km, and MPG definitions used to derive the L/100km formula(as of 2026-09-25)
NIST — Guide for the Use of the International System of Units (SI) — Authoritative conversion factors for gallons, liters, miles, and kilometers(as of 2026-09-25)