Over 500 kilometres, a car doing 10 miles per gallon burns 117.6 litres. At 20 mpg it burns 58.8. That upgrade saves 58.8 litres.
The same distance at 30 mpg takes 39.2 litres, and at 50 mpg it takes 23.5. That upgrade, worth twice as many miles per gallon, saves 15.7 litres, barely a quarter as much.
The number is the wrong way up
Miles per gallon measures distance per unit of fuel. What you buy, and what an engine emits, is fuel per unit of distance. Those are reciprocals, so equal steps in mpg are not equal steps in consumption.
Fuel used falls steeply at the low end and flattens at the high end. Improvements to the thirstiest vehicles are worth the most litres, which is exactly the opposite of the impression the mpg scale gives.
Litres per 100 kilometres reads correctly
Expressed as consumption, the same figures line up sensibly: 23.52 litres per 100 kilometres at 10 mpg, 11.76 at 20, 7.84 at 30 and 4.70 at 50. Halving the consumption figure always halves the fuel used, at every point on the scale.
This is why the metric convention is the more useful one for budgeting and for policy, and why comparing two cars in litres per 100 kilometres avoids the trap entirely.
Which gallon, and which figure
An 8 litres per 100 kilometres car is 29.40 mpg by the US gallon and 35.31 mpg by the imperial one, a difference of about 20 per cent from the unit alone. Comparing an American figure against a British one without conversion makes the wrong car look more efficient, which is the same class of error covered in the piece on unit conversions.
Manufacturer figures also come from standardised test cycles rather than from your commute, so treat them as a basis for comparison between cars rather than a prediction of your own consumption.
What it means in practice
When choosing between upgrades, compare litres over the distance you actually drive, not the headline economy figure. Replacing the least efficient vehicle in a household usually saves more fuel than making an already efficient one better.
The fuel economy converter moves between all four conventions, the fuel consumption calculator works from a real tank of fuel, and the fuel cost per kilometre calculator converts consumption into money.