Learn French: Le concessionnaire affirme que le kilométrage est bas. - The dealer claims that the mileage is low.

Even though affirmer (to claim / to assert) expresses someone's personal statement, it implies certainty on their part. Verbs of declaration and certainty take the indicative in French. You would only use the subjunctive if the statement were negative or a question (for example, Il n'affirme pas que le kilométrage soit bas).

English uses "mileage", but the French word is based on kilometers. How do you refer to the distance if the car tracks it in miles?

You still generally use le kilométrage. It is the standard term for the distance a vehicle has traveled, regardless of the unit of measurement. While you might occasionally hear le millage in Canadian French, in Europe, it is almost always le kilométrage.

What exactly is a concessionnaire? Does it mean any kind of dealer?

A concessionnaire is specifically an authorized dealer or franchise owner, and it is most commonly used for car dealerships (un concessionnaire automobile). For a general merchant or seller of other goods, you would use un vendeur or un marchand.

Could I use faible instead of bas here to mean "low"?