This is interesting... The calculator seems to be simply based on miles driven annually and EPA MPG numbers. I don't think it takes into account the cleaner burning of different types of engines, and how much cleaner/dirtier they may be.

I did a comparison of my old truck and my new FEH. The numbers were way too close - much closer than my real-world calculations. Mainly because it also doesn't take into account the type of driving - much of my driving is in an ultra-urban environment where you're at a stand-still a lot of the time. The actual mileage numbers for my truck were closer to half that of the EPA with this type of driving, while the FEH is at least close to them.