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).
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.