This a real anomaly - Red Diesel is only for use in farm vehicles BUT you can use it on the road if you want to take the chance of detection.
Marine Diesel is similarly low-priced and is intended for use in marine engines BUT you could use it in a diesel vehicle on the road too.
Avgas costs the earth but you can only use it in the air because if you put it in your car, the lead in it will poison the catalyst at great expense to replace it.
So the fuels on which it is possible to cheat, are low-tax, the one on which is difficult to cheat is highly taxed.
