Unit & Measurement Tables
Fuel Economy Conversion Table
Fuel economy is the only common conversion where the two units run in opposite directions. Miles per gallon goes up when a car improves; litres per 100 kilometres goes down. That inversion means the relationship between them is not a multiplication but a reciprocal, and a chart of one against the other is a curve, not a line.
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The reciprocal matters more than it looks. Going from 10 to 20 mpg saves 5 gallons over 100 miles. Going from 30 to 40 mpg saves less than one gallon over the same distance. In L/100 km the improvement is stated directly, which is why the European unit is the better one for comparing cars: a drop from 9.4 to 7.1 L/100 km is the same absolute saving as from 7.1 to 5.7, and the number makes that visible.
There is also the gallon problem. A US gallon is 3.785 L and an imperial gallon is 4.546 L, a 20.1% difference, so the same car is rated about 20% higher in Britain than in America with no mechanical change at all. Every table below states which gallon it means, and the third table converts between them directly.
US miles per gallon to litres per 100 km, km per litre and UK mpg
| mpg (US) | L/100 km | km/L | mpg (UK) |
|---|---|---|---|
| 10 | 23.52 | 4.25 | 12.01 |
| 15 | 15.68 | 6.38 | 18.01 |
| 20 | 11.76 | 8.5 | 24.02 |
| 25 | 9.41 | 10.63 | 30.02 |
| 30 | 7.84 | 12.75 | 36.03 |
| 35 | 6.72 | 14.88 | 42.03 |
| 40 | 5.88 | 17.01 | 48.04 |
| 45 | 5.23 | 19.13 | 54.04 |
| 50 | 4.7 | 21.26 | 60.05 |
| 55 | 4.28 | 23.38 | 66.05 |
| 60 | 3.92 | 25.51 | 72.06 |
| 70 | 3.36 | 29.76 | 84.07 |
| 80 | 2.94 | 34.01 | 96.08 |
The UK column is always about 20% higher for the same car, because the imperial gallon holds 4.546 L against 3.785 L. 40 mpg (US) is 48.04 mpg (UK).
Litres per 100 km to US mpg, UK mpg and km per litre
| L/100 km | mpg (US) | mpg (UK) | km/L |
|---|---|---|---|
| 3 | 78.4 | 94.16 | 33.33 |
| 4 | 58.8 | 70.62 | 25 |
| 5 | 47.04 | 56.5 | 20 |
| 6 | 39.2 | 47.08 | 16.67 |
| 7 | 33.6 | 40.35 | 14.29 |
| 8 | 29.4 | 35.31 | 12.5 |
| 9 | 26.13 | 31.39 | 11.11 |
| 10 | 23.52 | 28.25 | 10 |
| 12 | 19.6 | 23.54 | 8.33 |
| 15 | 15.68 | 18.83 | 6.67 |
| 20 | 11.76 | 14.12 | 5 |
| 25 | 9.41 | 11.3 | 4 |
Lower is better in this table, the opposite of mpg. That inversion is why comparing a European and an American figure by eye so often goes wrong.
UK miles per gallon to US mpg, litres per 100 km and km per litre
| mpg (UK) | mpg (US) | L/100 km | km/L |
|---|---|---|---|
| 20 | 16.65 | 14.12 | 7.08 |
| 25 | 20.82 | 11.3 | 8.85 |
| 30 | 24.98 | 9.42 | 10.62 |
| 35 | 29.14 | 8.07 | 12.39 |
| 40 | 33.31 | 7.06 | 14.16 |
| 45 | 37.47 | 6.28 | 15.93 |
| 50 | 41.63 | 5.65 | 17.7 |
| 55 | 45.8 | 5.14 | 19.47 |
| 60 | 49.96 | 4.71 | 21.24 |
| 70 | 58.29 | 4.04 | 24.78 |
| 80 | 66.61 | 3.53 | 28.32 |
A UK brochure figure of 50 mpg is 41.63 mpg in US terms and 5.65 L/100 km. Subtract roughly a sixth to get the number a US reviewer would quote.
Kilometres per litre to litres per 100 km and miles per gallon
| km/L | L/100 km | mpg (US) | mpg (UK) |
|---|---|---|---|
| 5 | 20 | 11.76 | 14.12 |
| 8 | 12.5 | 18.82 | 22.6 |
| 10 | 10 | 23.52 | 28.25 |
| 12 | 8.33 | 28.23 | 33.9 |
| 14 | 7.14 | 32.93 | 39.55 |
| 16 | 6.25 | 37.63 | 45.2 |
| 18 | 5.56 | 42.34 | 50.85 |
| 20 | 5 | 47.04 | 56.5 |
| 25 | 4 | 58.8 | 70.62 |
km/L is the unit used in Japan and much of Asia. 20 km/L is 5.00 L/100 km, which is 47.04 mpg (US).
How to Use These Tables
- Reading a US window sticker in Europe: divide 235.21 by the mpg figure to get L/100 km. 32 mpg is 7.35 L/100 km.
- Reading a European figure in the US: divide 235.21 by the L/100 km figure. 6.5 L/100 km is 36.19 mpg.
- Converting UK to US: multiply UK mpg by 0.8327, or divide by 1.2009. 50 mpg (UK) is 41.63 mpg (US).
- Estimating fuel for a trip: litres needed = distance in km × L/100 km ÷ 100. A 450 km trip at 7.5 L/100 km burns 33.75 L.
- Remember that mpg improvements are not linear in fuel saved. Going from 15 to 20 mpg saves more fuel than going from 40 to 50 mpg over the same distance.
Common Questions
Why is UK mpg higher than US mpg for the same car?
Because the imperial gallon is 4.546 L and the US gallon is 3.785 L. The same volume of fuel covers about 20% more miles when counted in the bigger gallon. Multiply US mpg by 1.2009 to get UK mpg.
How do I convert mpg to L/100 km?
Divide 235.21 by the mpg figure for US gallons, or 282.48 for imperial gallons. The constant is 100 × 3.785 411 784 ÷ 1.609 344, so it follows from the definitions rather than being a rounded rule of thumb.
Is 30 mpg good?
For a petrol car it is about average: 30 mpg (US) is 7.84 L/100 km, which sits between a compact and a mid-size sedan. A hybrid typically returns 50 mpg, or 4.70 L/100 km.
Why does improving from 40 to 50 mpg save so little fuel?
Because fuel used is the reciprocal of mpg. Over 100 miles, 40 mpg uses 2.5 gallons and 50 mpg uses 2.0 — a saving of 0.5 gallons. Going from 10 to 12.5 mpg saves 2 gallons over the same distance.
What is the difference between mpg and km/L?
Both measure distance per volume, so they are proportional: 1 mpg (US) is 0.4251 km/L. Multiply km/L by 2.352 to get mpg (US).
Does the table account for the difference between US and imperial gallons in L/100 km?
No, L/100 km is metric and has no gallon in it, so the figure is the same in every country. Only the mpg columns change with the gallon definition.
More Unit & Measurement Tables
Sources
- Fuel economy testing and the conversion between mpg and L/100 km — U.S. Department of Energy, Office of Energy Efficiency
- NIST Special Publication 811 — the US gallon and other non-SI volume units — National Institute of Standards and Technology
- Weights and measures — imperial gallon definition — National Institute of Standards and Technology
Filed with Everyday Life calculations.