That is of course a good point. You can make small amounts of those from agricultural waste. If demand outstrips supply from waste though, the area you need to

That is of course a good point. You can make small amounts of those from agricultural waste. If demand outstrips supply from waste though, the area you need to farm to drive a couple of miles is really large. I didn't do the math, but I wouldn't be surprised if you got more energy out of agricultural waste if you turn it into electricity+heat in a biogas setup.