# How to Convert Kilometers/Kilogram (km/kg) to Miles/Gallon (MPG)

I need to convert Kilometers per Kilogram to Miles per Gallon in order to calculate the Carbon Footprint of daily work commuters with vehicles using CNG (Nat Gas).

In order to do this, I need to be able to figure the density of CNG. But, I am unsure of the pressure. The commuters are driving Natural Gas Vehicles. (http://www.fueleconomy.gov/feg/bifueltech.shtml)

Not sure if this is correct, but I found this information: Mass density (kg/m³) = 0.712 from (http://www.unitrove.com/engineering/tools/gas/natural-gas-density)

The data entered by the users ranges between 8 and 20. Therefore, the result should come up with a similar number to miles per gallon.

• I shall migrate your question to Mathematics. Feb 10 '15 at 7:23
• You need to provide the density in kg/m^3 of the gas you are using otherwise it can't be done. If the CNG is pressurised that is the relevant figure.
– user117644
Feb 10 '15 at 7:35
• the density of compressed natural gas is mucn higher than tne numbers sbown here. you need to know the pressure. that said are you sure you dont mean to calculate equivalent mpg? en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent Feb 10 '15 at 12:35
• So just repeat the calculation below with the pressurized gas (ie mass) density to get the correct conversion factor and then multiply both sides of the equation by the figure for km/kg.
– user117644
Feb 11 '15 at 0:17
• You have measurements which are numbers 'in' km/kg (which actually means 'multiplied by' km/kg) and 'the equation' is the last line of calculations below with the section between the two equal signs taken out. So if you 'multiply both sides' it just means place the measured number in front of the km/kg on the left and multiply the same number by the 0.0016747... on the right. This results in the answer you want; and it is already 'in' (ie in front of) mile/gallon.
– user117644
Feb 12 '15 at 8:29

km/kg to miles/gallon (US)

1 km = 0.621371 mile

1 gallon = 0.00378541 m^3  therefore  m^3 = 1 gallon / 0.00378541

gas density = 0.712 kg/m^3 = 0.712 kg / (1 gallon / 0.00378541)
= 0.00269521192 kg/gallon


Set the above to 1, rearrange for kg and substitute:

km/kg = 0.621371 mile / 371.028338284 gallon = 0.0016747... mile/gallon


$$1\frac{\text{km}}{\text{kg}}=1\frac{\text{km}}{\text{kg}}\cdot\frac{0.712\text{kg}}{\text{m}^3}\cdot\frac{\text{mi}}{1.60934\text{km}}\cdot\frac{\text{m}^3}{264.172\text{gal}}=0.0016747\ldots\frac{\text{mi}}{\text{gal}}$$