Miles per gallon (MPG) is a standard unit of measure (a metric) that measures how many miles a vehicle can travel on one gallon of fuel. It is used similarly in North America and the United Kingdom, although the U.S. gallon is 80% of the Imperial gallon used in the UK. MPG.COM has no website.

Increase in MPG and decrease in L/100 km

One should note that MPG works differently than litres per hundred kilometres. L/100 km 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/100 km 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 gallons per mile) vehicle saves only 83 gallons for similar driving patterns.

Since L/100 km is linear instead of reciprocal, a driver who travels 15,000 km a year and switches from a vehicle with 20 L/100 km to one with 19 L/100 km has the same 150 L/year savings as someone who switches from 5 L/100 km to 4 L/100 km.

Because consumption is an inverse function of MPG, MPG can be a misleading indicator of fuel efficiency gains. People intuitively take the difference in MPG when comparing two cars. This leads them to underestimate the savings from small improvements on low MPG cars (e.g., 14 to 20 MPG, which saves twice as much fuel over a given distance as the improvement from 33 to 50 MPG, www.mpgillusion.com). A measure of "Gallons Per Mile" (GPM), such as gallons per 100 miles, provides an accurate view of consumption for a given distance of driving. Unlike MPG, the GPM of one car can be subtracted from the GPM of another car to get a direct measure of fuel savings.

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