Units of Measure
The two most common ways to measure automobile fuel usage are:
- units of fuel per fixed distance
- The amount of fuel used per distance, generally in liters per 100 kilometers (l/100 km), according the SI used in Europe, China, South Africa, Australia and New Zealand. Canadian law allows for use of either liters per 100 kilometres or miles per imperial gallon.
- units of distance per fixed unit of fuel
- Efficiency is defined as output per input. In automobiles it is the distance traveled per unit of fuel used; in miles per gallon (mpg) or kilometres per litre (km/l), commonly used in the UK, US (mpg) and Japan, Korea, India, Pakistan, Thailand, parts of Africa, The Netherlands, Denmark and Latin America (km/l). If mpg is used the gallon should be identified, since an imperial gallon is about 20% larger than the US gallon.
Fuel economy and fuel consumption are reciprocal quantities. To convert either way between l/100 km and miles per US gallon, divide 235 by the number in question; for miles per imperial gallon, divide 282 by either number. For example, to convert from 30 mpg to l/100 km, divide 235 by 30, giving 7.83 l/100 km; or from 10 l/100 km to mpg (US), divide 235 by 10, giving 23.5 mpg. To convert between l/100 km and km/l, divide 100 by the number in question.
In terms of dimensional analysis, units-of-fuel-per-fixed-distance fuel consumption has units of volume/length, which is equivalent to:
- volume/length = length3/length = length2 = area.
For example in meters, one liter is 0.001 m3, while 100 km=100,000 m, so l/100 km has units of
- 1/100,000,000 m2 = (1/10,000 m)2 = (0.1 mm)2 = (100 μm)2 = 0.01 mm2
Physically the area represents how thin a 100 km-long pipe or tube should be to contain one liter. The fuel-consumption area is the section of the tube. In meters 1 l/100 km = 0.01mm2. So if a vehicle consumes say 5 liters/100 km then that imaginary tube would be 5*0.01mm2 = 0.05mm2.
Read more about this topic: Fuel Economy In Automobiles
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