Been working through this issue about HHO being the same as #2 Diesel. My supplier is Southern States. Here's what they told me:
1) It all comes out of the same tank - it is chemically the same stuff. They get a tanker of Diesel and pump some into the the tanks for their retail road diesel business, with the rest going into the tank that gets dyed. The ONLY distinction in the product is the taxes that apply.
2) In KY there are 3 levels of taxation - Full (Federal & State), Off Road (State only), and Non-Taxed. Full is for stuff that is used on the roads. Off road might be used in construction equipment or other equipment, not running on black top. Non-taxed is for agricultural and HHO. FWIW Tractors are not automatically assumed to be employed in agricultural activities. For example, I live around Lexington, KY - Thoroughbred central - and lots of tractors here cut paddocks, and bale hay for horses. Horses are not considered to be an agricultural pursuit since we don't eat them, wear them, or use them in production of food (Yes, the Amish get a break on that last one, but then, they don't buy diesel...).
3) When you call and request a delivery of either HHO or off road diesel, the driver of the delivery truck verifies that you are using it for what you say, and charges you for that product. For example, if you order HHO and put it in an elevated tank, 1/2 mile from a house, he is likely going to charge you the KY tax. If you have it put into a tank next to a house, but you seem to be using lots of it in the summer (i.e. you are drawing from that tank to run your tractor), he will not likely charge the KY tax, unless you piss him off ;P
You can get a license from the revenue cabinet that will allow you to always buy the non-taxed stuff. You have to promise that it is used for agricultural activities. To them baling hay (for anything) is good enough! (Form 72A135 if you care).