It won't make any difference. Then why the complaint?
I suspect it will indeed make a difference. People will get a story going forward. There is already experience in the marketplace that labels like this work. Energy Star being the biggest example.
The fact is, that the labels do provide a wide variety of information. This along with features, price and lust will make a difference (If we were all about features and price we would all be driving Kia Amanti's but very very few people do).
There are a universe of new cars out there, and the car industry cannot be trusted to provide much objective information that can be compared to other cars. Information that may or may not be useful to all customers, but for some and I would propose a significant number of customers, they will look and understand the information. If presented in a way that makes sense.
I think the comparisons between all cars, like cars, and cars in its class pretty much provide the information that is most likely to be valuable.
If you don't think it will work, who cares? If you think it will cause harm, by all means speak up.
But I really do not think that you should limit the information about cars to that just limited by the manufacturer or the car dealer. To do so, actually does provide harm to the customers. And to expect them to all go to some third party doesn't make sense either. Only a small amount of consumers do that. That is why were are not all driving Ford Taurus's.