I changed the instructions on the add a car page, under description. Now it says: Optional details about your car. (eg. "VW Jetta SEL 5-cylinder 170 hp..."). That should at least give new folks a suggestion to include more info about their trim level if they know it.
Absolutely. I'm not saying that it's not useful info to know. In fact there are dozens of attributes you could track that we don't provide a way to track. We are erring on the side of simplicity, and we know that's not going to appeal to everyone. We also know that keeping something simple to use could mean more people using that thing. So that's what we're trying to do. If we provide a form that includes a dozen different switches about intricate details of someone's car, there's a good portion of the population that will start to think, "this place isn't for me."
Our top goal is to get as many people as possible tracking and thinking about fuel economy. The fact that we get some nice stats to look at for comparing vehicles is a nice side effect of that.