Convert fuel economy between L/100km, miles per gallon (US and Imperial) and km/L — the inverse relationship explained.
What Is a Fuel Consumption Converter?
A fuel consumption converter translates fuel economy — and handles its famous inversion: L/100km measures consumption (lower is better), while mpg and km/L measure economy (higher is better). The bridge constants: 235.2145833 ÷ mpg (US) = L/100km; 282.4809363 for Imperial gallons. So 8 L/100km = 29.4018 mpg (US) — and doubling economy halves consumption, the inverse relationship that makes “twice the mpg” not mean “twice as good” in fuel saved.
How to Use
- Enter the value.
- Pick the “from” unit — note US vs Imperial mpg.
- Pick the “to” unit.
- Click “Convert Fuel Use”.
Formula
Convert to L/100km first (mpg → 235.2145833 ÷ mpg for US, 282.4809363 for Imperial; km/L → 100 ÷ km/L), then convert out.
Worked Example
8 L/100km → mpg (US): 235.2145833 ÷ 8 = 29.4018 mpg. And 30 mpg (US) → L/100km: 235.2145833 ÷ 30 = 7.8405 L/100km.
How It Works, In Plain Words
Every conversion on this page passes through one hub: L/100km. Whatever you enter is first turned into litres per 100 km, then turned out into the unit you asked for, using two bridge constants and one reciprocal. From mpg (US), L/100km = 235.2145833 ÷ mpg; from mpg (Imperial), 282.4809363 ÷ mpg; from km/L, 100 ÷ km/L. So 8 L/100km needs no first step, and the way out to mpg (US) is the same bridge in reverse: 235.2145833 ÷ 8 = 29.4018. The Imperial constant is bigger because the Imperial gallon is 20% larger.
The inversion is the part to internalise. mpg and km/L measure distance per fuel — higher is better. L/100km measures fuel per distance — lower is better. Because they are reciprocals, equal mpg steps are not equal fuel steps: the savings FAQ shows going from 10 to 20 mpg halves consumption, 23.5 down to 11.8 L/100km, while 30 to 40 mpg only moves it from 7.8 to 5.9. That is why the introduction warns that doubling economy halves consumption — “twice the mpg” never means twice the fuel saved.
Real-World Uses
A buyer comparing an American listing at 30 mpg (US) with a European one at 7.8405 L/100km converts both to the same unit and discovers they are the same economy wearing two dialects. A British driver reading a US review checks the Imperial option before despairing: Imperial mpg runs about 20% higher for the same car, so the home figure is kinder than the imported one suggests. A driver tracking their own car in km/L converts 8 L/100km to 12.5 km/L — 100 ÷ 8 — to compare the dashboard readout against the figure in the handbook.
Benefits
- Four units with the US/Imperial split
- The inverse relationship explained, not hidden
- Both directions in one tool
- Car shopping and trip planning friendly
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device
Common Mistakes
- Treating mpg gains as linear savings — going 10→20 mpg saves more fuel than 30→40
- Mixing US and Imperial mpg (20% apart)
- Comparing L/100km as “higher is better” — it is lower-is-better
Common Mistakes to Avoid
The mistakes list names the three classics: treating mpg gains as linear savings, mixing US and Imperial mpg, and reading L/100km as higher-is-better. In prose, two more. Do not convert with the wrong bridge constant: 235.2145833 belongs to US gallons and 282.4809363 to Imperial, and swapping them builds the 20% error straight into the answer. And do not average mpg figures by adding and halving: because the relationship is reciprocal, the honest route is the tool’s own — convert each figure to L/100km, average there, then convert back.
FAQ
What is 8 L/100km in mpg?
29.4018 mpg (US).
What is 30 mpg in L/100km?
7.8405 L/100km.
Why do mpg and L/100km invert?
One measures distance per fuel (higher better), the other fuel per distance (lower better) — they are reciprocals.
What is the difference between US and Imperial mpg?
The Imperial gallon is 20% larger, so Imperial mpg figures are about 20% higher for the same car.
Which saves more fuel: 10→20 mpg or 30→40 mpg?
10→20 mpg — halving consumption from 23.5 to 11.8 L/100km vs 7.8 to 5.9.
What is 8 L/100km in km/L?
12.5 km/L. km/L is the reciprocal route from L/100km — the formula section gives km/L → 100 ÷ km/L — so the way back is 100 ÷ 8 = 12.5. The two units describe the same economy from opposite ends: one counts litres per 100 km, the other kilometres per litre.
Why are there two different bridge constants?
Because there are two different gallons. The US bridge is 235.2145833 and the Imperial bridge is 282.4809363, and the gap between them is exactly the point of the US-vs-Imperial FAQ: the Imperial gallon is 20% larger, so its constant — and its mpg figures — run about 20% higher for the same car.