Miles per gallon

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Miles per gallon (MPG) is a measure of fuel economy in automobiles used in North America and the United Kingdom

This is a metric that can mislead potential buyers of cars. Traditionally, it has been quoted under favorable conditions for the referenced car. While the EPA has made efforts to control this, it keeps showing up in variations. Such variations use the term MPGe which is a useful measure if applied correctly, as suggested under the Wikipedia article under MPGe. Recently the term MPG has been used in a misleading way in connection with "Plug-in Hybrid Electric" vehicles (PHEV). Miles per gallon when applied to such vehicles often only counts the actual gasoline gallons and ignores the energy from electricity, which is of course, dependent on a generating fuel source, a majority of which is coal. We also see a term "MPG+" which seems to also ignore the energy from the electric grid in the calculation of the quoted quantity, but does at least suggest that there is something more than gallons involved. Unfortunately, the facts are clearly spelled out in relatively obscure qualifications.

[edit] Conversion to SI units

Most countries other than the US and UK use the SI (aka metric) units litre (0.22 Imperial gallon or 0.264 US liquid gallon) and km (0,621 statute miles). These can be combined to either km/L (efficiency) or L/100km (consumption). The UK is a special case in this respect, as distances are measured in miles but fuel is sold by the litre. As a result, both MPG and L/100km are usually quoted for any given vehicle, although the general public almost exclusively use miles-per-gallon.

For US liquid gallons:

  • 1 L/100km = 235.21458 MPG
  • 1 MPG = 235.21458 L/100km
  • 1 km/L = 2.3521458 MPG
  • 1 MPG = 0.425 km/L

For Imperial gallons (UK):

  • 1 L/100km = 282.4809 MPG
  • 1 MPG = 282.4809 L/100km
  • 1 km/L = 2.8248 MPG
  • 1 MPG = 0.354 km/L

[edit] Increase in MPG and decrease in L/100km

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.