"Freight Best Practice confirms that an accepted methodology has been used in these calculations."
1 Gallon (Imperial) = 4.54609188 ltrs. 1 Mile = 1.609344 kms. Diesel:CO2 conversion of 2.63 kgs (0.00263 tonnes) per litre. Source: DEFRA.
Whilst CO2 is directly proportional to litreage consumed, NOx and particulate emissions vary depending on engine and exhaust management systems.