Is it prescribed for a Muslim to recite the supplication for riding after having driven a distance in the car, or should he omit it because he did not begin with it at the time of mounting?

It is recommended to recite supplications upon mounting an animal or a vehicle. This should be done when one is settled on the back of the conveyance, based on the hadith of Ibn Umar (may Allah be pleased with them both), who narrated that when the Prophet (peace and blessings be upon him) settled on his camel for travel, he would say "Allahu Akbar" three times, then say: "Glory be to Him Who has subjected this to us, and we were not ourselves able to subdue it. And verily, to Our Lord we shall return..." etc. If one forgets the supplication at the time of mounting and then remembers it after having covered some distance, there is no impediment to reciting it as long as one is still riding.