"Vehicles" is a tag without definition. When I tried to fill it in, I found that the description I wanted to use was basically already used by "transportation". So I looked at the questions.
The thorium-powered steam car is under "transportation". Could a photonic engine actually work? is under "vehicles". etc...
Looking back and forth between the two, the questions that these tags have been used for are pretty much interchangeable.
I recognize that there is some broadness to transportation -- there's one about sand worms, for example, that would possibly not be under "vehicles". But I still propose consolidating these into synonyms for each other, and making "transportation" the primary tag.