As most say, it is a lot of hype, and trying to get us less dependent on fossil fuels. I was a diesel mechanic at one time, so have studied this quite a bit. Bio diesel is OK, IF you have the fuel system that is compatable with it. Woe to those who try to run it in an older system, you will be looking at a lot of money. The "bio" in the diesel is an excellent cleaner/detergent, just like the "alcohol" is in gas. But, too much "bio" in the diesel, and the old system seals are not designed for it. Bye bye fuel pump, injectors, etc. A fellow I know started running bio diesel in his older Volkswagen. About 3 months, and suddenly fuel was leaking everywhere. $1400 later, he could run bio.....with his NEW bio compatable system. With a small amount of "bio" in the fuel, it shouldn't be a problem, and shouldn't impact fuel mileage much. The "bio" has a bit lower "cetane" rating (cetane rating being similar to gasoline's octane rating). The higher the cetane rating, the more BTU's (power) from the fuel. If you are a farmer in the midwest, making your own bio fuel, the benefits are obvious!!!
The other drawback to bio diesel is it's lack of low temperature flow characteristics. A couple of years ago the loco transit agency tried using it, and it was fine, until we had a cold spell. Half the fleet wouldn't start. They had to leave the engines running all night to keep the fuel warm and flowing.
