gramatická analýza: Viděl rychle jedoucí auto.

Present active participle from jet (3pl. jedou plus -cí)

The main verb (the conjugated verb) is viděl; the subject of this sentence is therefore on; the direct object is auto; the PrAP that modifies the auto therefore is in the accusative neuter singular.

Try paraphrasing the sentence as "He saw the car which was driving fast."