One should note that MPG works differently than litres per hundred kilometres. L/100km denotes a rate of fuel consumption, while MPG is a measure of fuel economy (or 'gas mileage'). If a car uses less fuel, the MPG increases, and L/100km decreases, but the percentages will not match, because the values are reciprocal.
For example, 20% better mileage does not mean 20%, but 16.7% less fuel. This comes from the following calculation: 20% is 1.2 times bigger distance, therefore 100% / 1.2 = 83.3% of the original fuel consumption, or 16.7% less fuel.
If a driver who travels 15,000 miles a year switches from a vehicle with 10 mpg to 12 mpg average fuel economy (0.10 gallons per mile to 0.083 gallons per mile), 250 gallons are saved. A similar 20% improvement in exchanging a 30 mpg for a 36 mpg (0.033 gallons per mile for 0.27) vehicle saves only 83 gallons for similar driving patterns.
Since L/100km is linear instead of reciprocal, a driver who travels 15,000 km a year and switches from a vehicle with 20 L/100km to one with 19 L/100km has the same 150 L/year savings as someone who switches from 5 L/100km to 4 L/100km.
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