Your carbon footprint is the sum of all emissions of CO2 (carbon dioxide), which were induced by your activities in a given time frame. Usually a carbon footprint is calculated for the time period of a year. — TimeForChange.org
The best way is to calculate the carbon dioxide emissions based on the fuel consumption. In the next step you can add the CO2 emission to your carbon footprint. - Time For Change.org
The EPA provides an online calculator for estimating personal emissions.
You can check the fuel economey of your, or any car at FuelEconomy.gov