Found on a TDI forum
It really depends on the State.
The federal road tax is 24.4 cents per gallon. To pay this, you must file form 637 to register with the IRS, then file form 720 quarterly to pay the tax.
There is probably also a state diesel road tax as well, although Texas (for one) exempts biodiesel, and vegetable oil.
There is no 400 gallon per year/quarter exemption, contrary to some posts.
Also, if you run straight WVO, there is a 50 cent credit available to offset the road tax. If you register as a blender (most people can't), you can get the extra 25.6 cents back from the IRS. If not, you will wipe out your road tax using the credit.
The 50 cent credit is NOT the biodiesel credit, which is only available to those registered biodiesel producers that meet ASTM specs and have full registration. There is no such credit for homebrewers.