I found a neat Flash demonstration of how much CO2 is produced from various fuel types produced by Volvo.

I think there are three types of "environmentally conscious" fuel consumers out there, and this site illustrates them.

The first type are the "dollars per gallon" consumers.
The second type are the "miles per gallon" consumers.
The third type are the "kg of CO2 per km" consumers (or the pounds of CO2 per mile, if you must go with Imperial units).

If you think about it, approaching this from the "dollars per gallon" biases the evaluation on an economic/infrastructure axis.

Why is gas often the cheapest fuel available? Because of the untold billions of dollars of investment and infrastructure that already exists (from oil wells, to pipelines, tankers, refineries, gas stations and vehicles in the marketplace that can use it).

Recently diesel has been looking pretty good. Modern diesels don't pollute much more than gas versions, they get considerably better miles/gallon, and to top it off, diesel costs less than even regular gas does.

What the Volvo simulation shows is how important it is to not only reduce the emissions of what is in your tank, but the emissions of what it took to get it there.

The promising news is that the top best-performing technologies can be reached from investments in lesser technologies. For example, Hydrogen cars can perform near the top or near the bottom of the list, depending on where you get the electricity to produce it-- getting hydrogen cars into the marketplace based on the ubiquity of available electricity and then cracking the nut of getting greater solar or wind power to power them could be a strategy to get a hydrogen infrastructure in place. Similar strategies are available for diesel engines (today they might burn diesel, tomorrow biodiesel).

Check out Volvo's CO2 Comparison and see for yourself.