[gasman]

People can be generalized into 2 major categories. Those who do research, and those who compulsively buy. Those who do research are going to know the truth anyway. Those who don't wouldn't anyway, and will still buy impulsively just like they do now.

They only need to change it for vehicles that don't use gas/diesel (liquid fuel). Perhaps it could be kw/h per mile? That wouldn't help with cost, but it would let vehicles be directly comparable in efficiency, which is what the crux of all this is anyway. If whatever they decide is not a standardized "unit of energy" per "distance" measurement, then it will simply be a meaningless label that confuses people (because people will assume that it has meaning, when it does not). For environmental awareness, some sort of greenhouse gas or CO2 rating could be added, but it is absolutely imperative that this rating be completely separate from the efficiency (energy/distance) rating. The reason they must be separate is because they measure two completely different things. They simply cannot be combined into a single meaningful number/label.

The only way they could do anything of the sort meaningfully would be to have a label or number that conveys two meanings at once. One option could be, for example, to have a kw/h/m rating that is big and colored. The color would correspond to the environmental impact of the vehicle. The scale would go from red to orange to green (in several steps- perhaps 10). It could then have a smaller number for the environmental impact (to quantify it, and for people who are color blind). The point would be, though, that anyone could tell from a distance the environmental impact and the efficiency. This "combined" label would be valid, reliable, and easy to comprehend. There would, of course, need to be some more information that goes along with it, in tables.

edit: why not go the way of electric appliances? They have an energy consumption rating scheme that not only shows the efficiency and estimated cost of yearly operation, but it also puts the number in perspective on a line, so you know how well it stands up to competition. This would work very well when combined with the above number/color label. It would probably be best to have the color (and associated impact number) relative to similar vehicles, but also have the numbers to make absolute (ratio) comparisons (and for knowledge of overall impact).