Its not a tax on the manufacture of it, its a tax on using it on the road. The gas station pump price has the road tax added in so when you make your own your not paying that tax. They only tax if your driving on the road, for example my parents buy diesel for their farm tractors and they tell the gas station ahead of time and don't have to pay the road tax.

I'm trying to find a way to go straight from crop to biodiesel, everything I find on it tells how to convert from vegetable oil so I'd have to make vegetable oil then convert to biodiesel, I'm wondering if there is a more direct method.