If you've done a vegetable oil conversion, or built a vegetable oil car from the ground up (like I did with MAX), or make your own biodiesel from waste vegetable oil, you still have a legal (and ethical, IMHO) responsibility to pay your per-gallon fuel tax. But depending on where you live, it may not be easy.
My native Oregon has a well earned reputation as a progressive state (with some embarrassing late starts—seriously, we didn't ratify the 15th Amendment until 1959) and one area where we've been ahead of the pack is road taxes. What that? You don't like road taxes? You don't like the fees for vehicle registration? You don't like gas taxes? Oh yes you do, because that's what pays for the infrastructure that makes driving possible—the asphalt, the bridges, the traffic lights—and one side effect of improving fuel efficiency is a reduction of road tax revenue.
Back in Ye Olde Days (the early 1900s) the biggest supporters of road taxes were car owners and auto clubs. The horse-and-buggy folks were pretty content with what they had, but “automobilists” wanted better roads and (for the most part) recognized that the users should be the ones to pay for them. And hey, us Oregonians led the way, with a gasoline tax in 1919, and a dozen years later, the feds decided that was a good idea and initiated a national gas tax. We also have a weight-mile tax for over-the-highway trucks, since an 80,000 pound 18 wheeler put more wear on the road than a fuel tax would cover.
We humans are pretty good at rationalizing why what's good for us personally is The Right Thing To Do. I'm quite comfortable with fuel tax, myself, and feel like I'm paying My Fair Share at the pump even though it's about a quarter of what other folks typically pay per mile. My justification? MAX's road maintenance costs are about a quarter of a typical car; MAX only weighs 1300 pounds and only has 32 horsepower, it's sure no asphalt wrinkler.
Alternative Fuel User
Nevertheless, something should be done about alternative fuel vehicles, and I'm not ready to say “Gosh, maybe the electric guys should be paying their share of road tax,” (which I personally think should be higher than MAX's share because the EVs tend to be heavier and more powerful than MAX and thus do more damage to the roads—see what I mean about rationalizing?) until I can claim the moral high ground. And I can't claim it yet, because just like EV drivers in Oregon pay no fuel tax when they plug their car into the wall, I pay no fuel tax when I buy cooking oil from Costco or 7-11 or Safeway or Kroger's.
When it goes in MAX's tank, it turns into what the Oregon Department of Transportation calls a “use fuel” (“use”, noun, with 's' pronounced like in snake, not like the s in bees), and the ODOT has a system for that, and they were happy to send me the necessary applications so I could get an official Use Fuel User License. And mail in my fuel tax every month. Plus send them a deposit in lieu of bond should I fail to mail my monthly fuel tax. And request a Use Fuel Vehicle Emblem (see the graphic at the beginning of this article, which is just the emblem header so don't get any ideas about photo shopping your own Emblem).
The paperwork, though weighty, is no tougher than an 8th Grade book report...but it's tailored for big fuel users and ODOT doesn't have a lot of 100 MPG grease burners on the books so the default deposit is a hundred bucks. Gulp. I sent them some eloquent emails and a link to a Mother Earth News MAX story, and they decided a tenner, which is good for 33 gallons of veggie oil, would be enough to keep me honest.
So we'll see how this goes. I'm going to have to keep fuel use and travel records much like a big rig trucker, and mail ODOT a monthly check, but I'll get to sleep without a guilty conscience so it will all be worth it. Besides, I can hardly wait to take my Use Fuel Vehicle Emblem to Safeway.