You can find out how many grams of CO2 per kilometer your car emits in the manual or in the documents of your car. If you don't it there, you may also try to find it in the green guide to car CO2 emissions. However, keep in mind that these values are correct only for new cars and under standard driving conditions. In reality, your car's emission rate will probably be higher than these official values. You can improve the accuracy of the estimations made by this extension by adjusting the emission rate in its options page.
If you do not mind a bit of inaccuracy, you may also use the following rough estimations: