Miles per gallon

Miles per gallon (MPG) is a metric term 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 Imperial gallon used in the UK is about 20% larger than the U.S. gallon.

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. Note that because the imperial gallon is significantly larger than the U.S. gallon, MPG figures are 20.095% higher in the UK than in the U.S. for the same real fuel economy.

US liquid gallons

*1 km = 0.621 Mile
*1 Mile = 1.609 km
*1 U.S. Gallon = 3.79 Litre
*1 Litre = 0.264 U.S. Gallon

*1 km/L = 0.621M/0.264G = 2.35 MPG-US
*1 MPG-US = 1.609km/3.79L = 0.425 km/L
*1 MPG-US ≈ 2.35 L/km

*1 MPG-US ~ 235 L/100km =>10 MPG-US ≈ 23.5 L/100km; 20 MPG-US ≈ 11.75 L/100km; 30 MPG-US ≈ 7.83 L/100km etc...

Imperial gallons (UK)

if 1 Litre = 0.22 Imperial gallon then

*1 km/L = 2.8248 MPG-Imperial
*1 MPG-Imperial = 0.354 km/L
*1 MPG-Imperial ≈ 282.4809 L/100km

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.

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.