Since you asked, here are my notes on the subject.
The estimated transportation fuel and home heating oil used in the United States is about 230,000 million US gallons (870 million m³) (Briggs, 2004). Waste vegetable oil and animal fats would not be enough to meet this demand. In the United States, estimated production of vegetable oil for all uses is about 23,600 million pounds (10,700,000 t) or 3,000 million US gallons (11,000,000 m³)), and estimated production of animal fat is 11,638 million pounds (5,279,000 t). (Van Gerpen, 2004)
Rapeseed = 110gal/acre
Fuel used in the US:
139.9 billion gal gas in 2005
38.3 billion gal diesel in 2005
38,300,000,000 / 110 = 350,000,000 acres needed to replace diesel usage with rapeseed based bio-diesel.
Total farmland in production in 1992 in US is 435,000,000 acres (USDA 1992) Note this was in '92 see below for current acres in production. ( I don't know what amount good farm land is available but out of production, I do know, however, that vast tracts of good farm land are being converted to developments all the time. My numbers are rough but my only goal is to put the magnitude of the issue in perspective.)
These are very rough numbers that don't take into acount other petroleum products besides motorfuels and may not take into account home-heating fuel.
http://www.fas.usda.gov/info/fasworldwide/2006/07-2006/BiofuelsOverview.htm
http://www.nass.usda.gov/census/census92/atlas92/html/m081.htm
Total planted farmland down to 318,610,000acres in 2006 according to --
http://usda.mannlib.cornell.edu/usda/current/Acre/Acre-09-12-2006.txt
Now you-all can do what ever calculations you want.
As you can see, since there is a shortfall in available acerage for growing bio-diesel, then 5% of the crop land,if used to grow rape seed, would yeild less than 5% of the total diesel currently used.
Now, there are other potential oil producing crops that have a higher oil yeild, but the type of farm land suitable for growing them is much more limited. If only those lands were used to produce only high yeilding crops, then the equation would be different. But the bottom line is still that bio-diesel cannot be considered a replacement for petroleum but only a suppliment.