When the El Camino is pushed over the cliff, it is obvious that I.S.A.A.C (Internally Sophisticated Analyzed Anhancement Syborg) has already jumped out of the car.
As the El Camino is going over the cliff, it is seen to be in perfect condition in the rear, supposedly after being hit in the rear by a high explosive round with enough effect to disable it.
At chapter 3 MIN15.34.22 Assistant Deputy Sheriff of the local law enforcement unit makes a clear attempt of putting his hands in his pocket, while in the next scene his hands are clearly in his pocket.
When the El Camino is pushed over the cliff, it is obvious there are no people in the car. Also, as the car turns over (in slow motion) there is no drive shaft - so it couldn't have been driven shortly before being pushed over the cliff.
The El Camino is driven to the cliff. When it falls off the cliff, just before it hits the ground you can see the entire undercarriage. There is no drive shaft, no transmission, and no engine therefore it couldn't have been driven.