Considering that not all vehicles are cars, I went ahead and tagged a number of questions with cars, a topic I'd argue is important enough to warrant its own tag, as opposed to the more generic vehicle, which would include bicycles, motorcycles, boats, aircraft, which would probably be more suited for vehicle intercomparisons.
I think Nate's suggestion to change the 'vehicle' tag to 'automotive' is good but I'm not sure if automotive is a term that is commonly known to non-native English speakers so I'd rather change it to motor-vehicle.
I've posted this as an answer so people can vote on and we can see which tag people prefer.
I'd support keeping the tags separate. This pedal powered car is neither a motor car nor an automotive, yet it is clearly still a vehicle.
From the perspective of a user who might search the site, tags like electric-vehicle or renewable-transportation may help distinguish between different types of questions. Depending on the question, one of these may be better than vehicle.
It's a shame actually that we don't have a tree structure of tags, as that would greatly help provide logical links between similar subjects...
I personally would consider
automotive to be a better term, for the types of questions that concern both cars and trucks (fuel efficiency, alternative fuels, engine technologies, etc.)
I have a hard time seeing how
vehicle is really a very meaningful tag, as there isn't that much that boats, trains, planes, cars and trucks have in common.
As I think there's only 5 questions in the vehicle tag, it might make more sense to migrate this to
automotive sooner rather than later, before site traffic increases much.
Disclaimer: I'm a speaker of American English, so if that term isn't equally as applicable in other English speaking countries, I'll gladly retract my suggestion.