Take speed into account
The CO2 emissions (and fuel consumption) of a car depend significantly on its speed. For example, a car driving at 150km/h on a highway consumes more and emits more CO2 per kilometer than a car driving at a low speed in a city without traffic jams.
The options page should allow the user to specify the emissions or fuel consumptions for 2 or 3 different speeds. The actual average speed of a route suggested by Google Maps can be calculated by dividing the route's distance by its time. Based on the average speed and on the emissions/consumptions for the speeds provided by the user, we can calculate more accurate estimations.