OK well I love this topic as most of my friends ... and actually every person in Europe are mad about mpg... I thought of calculating environmental impact not per car but per person sitting in the car:
1 liter of gas burnt = 2 kg of co2
I use 0.17 liters of gas per 1km
that is 5.8823 units per liter
2/5.8823=0.34kg of co2 per 1km
conclusion my Chevy Van emits 340gCO2/km (cat G*)
a Toyota Prius emits 104gCO2/km (cat A*)
Now for the fun part
Prius seats 5 adults so the impact is 104/5= 20.8g of co2 per person per km
My Chevy seats 12 so the impact is 340/12= 28.333 of co2 per person per km
This means that the Chevy has an impact equivalent per person to a car emitting 141.6g of co2 per km
*This article might add extra info:
http://en.wikipedia.org/wiki/ACEA_agreement
Seriously I run at an average 14 - 15 mpg 50/50 Town n Country
OK calculation are approximate and I am not even sure as co2 emissions seems to be quite a dark art ... wonder why