But because of the rotation of the Earth and in fact the whole Galaxy in the meantime, the vertical component of the ravine became the horizontal component of the road :)

If we assume time travel is possible which is quite a leap already, why do people fixate on things things having to keep their fixed point in space relative to... well.. relative to what? Frames of reference can really complicate this.

Perhaps time and gravity are related in a manner that means as an object travels through time it maintains position relative to gravitational influences (and local ones affect it far more than others due to the inverse-square relationship) and maybe the effect is strong enough on each part of the travelling body that it maintains orientation relative to those gravitation influences too (so the car not only stays on the road but conveniently wheels-down as well).

You only need to die instantly in that timeline.

1. Write yourself a note to keep a jerry can in the Delorean, put it on your person.

2. Set destination time back by a week.

3. Marty gets in Delorean and Doc pushes it off of cliff.

4. Next of kin and Marty read note given to them at the morgue.

5. Plot of BttF 3 evaporates.

Edit: This is too complicated. Just visit Western Union again and send a second letter about the jerry can.

Edit 2: Wait, there should be a 2nd Delorean in 1885 after Marty arrives. So Doc's Delorean should have some fuel unless Doc was joy-riding. I'm really not thinking 4th dimensionally.

Hadn't thought of that! And considering that based on what I calculated in the post he goes off the edge at about 70mph, he probably needs way less that 258 feet right?

The 70mph is horizontal velocity, gravity is vertical acceleration from 0. Total velocity is sqrt(sum of squares). It's unclear how much horizontal velocity the train would lose due to air resistance and due to the unclean launch from the ravine. (Would it fly off the end of the track, or tip off the end and tumble down?)