What does the description of Montag's knee as being hit by a car really mean?
This description comes from Part Three of the story when Montag is attacked by the Mechanical Hound:
He felt all of the mingled relief and horror at having pulled back only in time to have just his knee slammed by the fender of a car hurtling by at ninety miles an hour.
Montag has not really been hit by a car. In fact, the Mechanical Hound has just injected Montag's knee with procaine, which is a type of anesthetic it uses to paralyze its victim.
By comparing this injection to being hit by a car, Montag is giving the reader a sense of how this injection felt. It is clear, for example, that he is overcome with pain and discomfort before the anesthetic kicks in and he is left with an incredible sense of numbness. In addition, this comparison gives us an idea of the Hound's power and strength. It is so strong, for instance, that its impact is like being hit by a car.
Fortunately, Montag is able to escape the Hound and, in fact, kills it with his flamethrower. The paralysis, however, slows him down and means that he must work twice as hard to avoid capture by the authorities.