This is a guess, but I think diesel has 20% more energy per unit
volume that gasoline, so 20% more carbon. A 60 mpg diesel puts the
same amount of carbon in the air as a 50 mpg gasoline car.
I think about this because I have a gas car that gets 33 mpg, and my
300 TDT, with its newly remanufactured engine, gets 33 mpg also. It's
a lot more comfortable and restful to ride in the Benz all day than it
is to ride in the Corolla. Moneywise they cost roughly the same, but
the Benz has the larger carbon footprint.