Here's why:
* Relative Clause: It's a clause that modifies a noun (in this case, "map"). It uses a relative pronoun ("that") to connect to the noun it modifies.
* Restrictive: It provides essential information to identify which map Smithers read. Without this clause, we wouldn't know which map.
The sentence implies that there are multiple maps, and the clause "that was car" specifies which one Smithers read.