Consider a post-Apocalyptic society of collective farmers that has access to a waning supply of modern farm supplies, a small, independent power grid, and a enough knowledge to keep themselves fed reasonably with only a few acres of farmland. These people are now considering acquiring more land for growing sunflowers from which to produce biodiesel fuel. Under these consditions, how many sunflowers do you need to produce 1 liter of biodiesel fuel and how much farmland would that equate to?
The National Sunflower Board states that sunflowers can produce 600 lbs of oil per acre, which translates to approximately 300L of oil per acre. This paper by the Swedish Royal Institute of Technology suggests that biodiesel can be created from vegetable oils at about 95% efficiency using transesterization with ethanol (which is fairly easily produced). This crop guide suggests that you can get about 15-25000 sunflower plants per acre.
A little math gives us about 1L of biodiesel from ~70 sunflower plants @ 20k plants per acre and a yield of 285L per acre. That's somewhat optimized, so you can build in factors of error as you like (maybe your farmers can only manage 50% yield efficiency).
The Corporate Extension group cites a production rate of 66 gallons of bio-diesel per acre for soybeans, and between 75 and 240 gallons per acre of bio-diesel for rapeseed and canola.