I should think that you have a case. I find it hard to believe that “Fuel Efficient” is defined in any great detail in any law or whatever might be applicable here. Hybrid, as you say, would be definitive either/or but fuel efficiency could be argued for almost anything. The largest monster truck could be fuel efficient compared to the same truck with less than perfect tire pressure… maybe not the best analogy. At least any car that is not a monster could be, in some context, considered fuel efficient. Stupid sign really.