Federally, you can brew up to 400 gals PER PERSON PER QUARTER without paying federal road taxes.
Each STATE however is different, some are following the federal view of bio-diesel, and some treat it as an additive (which is not taxed), still others want their money.
Even if your state wants the money, most of them dont have a system in place to collect! So even if you WANTED to pay, they wouldnt know where to send you.
One thing that has been suggested, is pay a quarterly tax on what you expect to use. Then, at the end of the year, file for a return of what you didnt use. Document everything. This way, if they ever try to nail you for running untaxed fuel, you can proudly show that you mail in a check every quarter, and that the state seams to have no problem cashing it! Burden is now on the state, and not you
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