When Sentinel Prime was founded on the Moon, he was already in the vehicle mode of an earth fire rescue truck. This is a widely recognized bug. But just now I found a possible explanation for this. Both Cybertron and Earth are inside the Milky Way Galaxy, and there are many planets other than these two which have progressed civilizations. When Sentinel took of in the Ark from Cybertron, he was awake and the Ark was well-functioned. But when he was founded on the Moon, he was in stasis with the Ark broken. So, there are many possibilities on the way. A fire rescue truck's design is functional. As long as a civilization has the function of fire rescue, the fire rescue truck would be designed that way or similar. Had the Ark had some short stop on other planets? The movie doesn't say "no" either. So the Ark may have some short stop on Xen (Half-Life), Sera (Gears of War), the Halos (Halo series) or whatever planet you like to think about. Though those planets are from other movies/games, but all these movies and games (including Transformers) are of the same kind of sci-fi world view. We can focus on smaller bugs, also can patch up them with a broader view.