From what I have read, if you are running home brewed bio in a road vehicle, in order to be strictly legal, you are SUPPOSED to keep track of how much you use and pay Federal taxes on it. Same for State taxes - though some states give you an exemption from the tax on the first X number of gallons - some its 200, others its 400, but it varies state to state.
Are they going to come after you for it? Not likely - unless the busybody neighbor turns you in, and even then they aren't likely to do more than send you a letter "informing" you of the laws. However, if you're brewing it on your premisis, and the neighbor complains, you better make sure you are following all the local Haz Mat storage regs, fire codes, etc. - just in case you do get a visit from "THE MAN".