Green Car Congress has an interesting post today looking closely at the relative performance and environmental stats of the 2005 model year Honda Accords: the four and six-cylinder gasolines, the four cylinder diesel, and the six cylinder hybrid. The results are quite interesting:
the Accord Diesel (using petroleum diesel) offers the lowest fuel consumption and the lowest CO2 emissions, even surpassing the Accord Hybrid.
The Accord Diesel (which is not offered in the US) gets 43.3 miles per gallon and emits ~143 grams of CO2 per kilometer, compared to 33/165 for the Accord Hybrid. Use of biodiesel would further lower the carbon footprint of the diesel Accord. And while some of the efficiency comes from being a four-cylinder instead of a six-cylinder vehicle, it's worth noting that the diesel model greatly out-performs the four-cylinder gasoline model across the board.
Be sure to read the comments; Mike also addresses questions about the relative non-CO2 emissions.
Here in Europe (well, Finland), we have Accords available with 2.0 and 2.4 litre gasoline engines. The diesel is the same 2.2l, and no hybrid is available here (AFAIK). I would consider even these engines a bit large by local standards - common diesels in cars of that size would be around 1.9-2.0 litres. We have V6's but they are generally considered an overkill, and mostly limited to premium cars (BMW's etc.). Renault is downsizing its diesels and now offers cars with a 1.5 litre 100hp engine, while new Ford Focus will be available with a nice 1.6 litre diesel.
I guess my point was that as you don't have the most economical versions available and we don't always have the most sporty versions available, it is easy to underestimate the differences in economy.
you can get diesel jettas in the US for around $20,000 :D
"100hp 1.9L SOHC 8 valve intercooled turbo I-4 engine with direct diesel injection... Fuel Economy City/Highway: 38/46mpg"
dunno about grams of CO2 or non-CO2 emissions/km tho...
From the EPA site:
2005 Jetta Air Pollution Score:
Diesel (non-California) = 4/10
Gasoline (Callifornia) = 10/10