The figure for the amount of CO2 saved relates to trips that have been logged for transport purposes only. The statistic relates to the total amount of CO2 released by an average car, of unknown fuel type, which roughly estimates the amount of CO2 saved by cycling instead of driving. It is calculated as follows:
Total units traveled x kg CO2 per unit = total kg CO2
This calculation is the total distance of trips logged for transport purposes multiplied by the amount of CO2 released by an average car, which equals the total kilograms of CO2 per mile.
DEFRA's carbon conversion factor for an average car fuel unknown was used. This is 0.29143kg CO2 per mile (0.18109kg of CO2 per kilometre) is provided in 'Greenhouse gas reporting: conversion factors 2017’.