That's true, there doesn't seem to be a mechanism in place for it. It will be interesting to see what happens with taxes as more and more cars are electric.
How about ethanol? Is the per gallon tax on gasoline also applied to the 10% ethanol in it? What if one produces their own ethanol - is any fuel tax imposed with the registration of the distallation equiptment? (Honest question, I have no clue)
There's taxes and ATF permits up the wazoo on distilling your own ethanol for fuel purposes, even if it's denatured to deny (safe) human consumption. And the ATF's own FAQ page states a gradeschool kid can't ferment potato peels and dinner scraps, and run a little tabletop steam engine off of the alcohol for a science fair project legally w/o the proper permits.
They regulate the hell out of any ethanol used in any industry, even if it's not intended for human consumption. At least at the manufacturing and distribution level. There's even permits for places that make flavoring syrups and extracts that use large volumes of alcohol as the carrier, like for vanilla extract etc.
If it's not denatured with methanol or isopropyl alcohol, they regulate it.
http://www.ttb.gov/industrial/forms.shtmlAnd they regulate, and require you to report the denaturing was done first. Only then is it sold in a non-regulated manner (from their standpoint) as industrial solvent or fuel. Then I'm sure state DOT's or the EPA have something else to say about it all after that...
As to woodgas on public roads, I don't think fuel tax would be the main issue. As stated, 100% EV's don't have legal issues (yet) with that.
State vehicle safety, registration, and emissions laws would probably be the main laws that
might bar the legal use of a woodgas vehicle on public roads. Some states it might be easier than others to "just do it" assuming the truck or car body's VIN was intact and had a current license plate and you weren't in a region that requires emission testing to get renewals. Although you might still be running afoul of regulations that require you to use all your vehicle's emission control systems as designed originally, or DOT approved aftermarket equivalents, if you were a DIY hot-rodder type etc.
However, perhaps a vehicle that's old enough to be grandfathered out of those regs would do. Dunno.
I think largely, the number of folks running woodgas vehicles is low enough, and they tend to be rural enough it's just a non-issue for now.