MOVES calculates the energy it takes to operate a vehicle. Then using the energy content of fuels, it can calculate the gallons of fuel necessary to operate the vehicle and/or the CO2 generated by burning the fuel. Both of these depend on the energy content of the fuel and the fuel density. MOVES has default average values for these fuel properties in the FuelType table in the default MOVES database.

grams of CO2 per kilojoule of energy = 1 / [ (kilojoules/gram of carbon) * (weight fraction of carbon in carbon dioxide) ]

The weight fraction of carbon in carbon dioxide is [12 / (12 + 16 + 16) ] = 0.273, where carbon has a molecular weight of 12 and oxygen has a molecular weight of 16.  A carbon dioxide molecule contains a single carbon atom and two oxygen atoms.

All of the carbon in the fuel is assumed to convert to CO2. A small portion of the fuel is initially released to the atmosphere as unburned fuel or as carbon monoxide (CO), but we assume that eventually nearly all of the released carbon will convert to CO2 in the atmosphere. This is why we refer to the CO2 as "atmospheric."

Some pollutants other than CO2 contribute to climate change. These other pollutants have an effect that is greater than that of CO2 and is often referred to as "CO2 equivalent" in units equivalent to the effect of CO2. The Pollutants table in the MOVES default database contains the global warming potential assumed for the pollutants included in MOVES.

Methane (CH4) has a value of 21.
Nitrous oxide (N2O) has a value of 310.
This means that a gram of methane has the same effect as 21 grams of CO2.