According to my calculations, we'll reach 88 miles per hour just before we hit the edge of the ravine, at which point we'll be instantaneously transported back to 1985, and coast safely across the completed bridge.
11 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:47:41.743 in the DVD version of the movie.
Quote context
[...]
- I apologize for the crudity of this model, but I just...
- Yeah, I know, Doc. It's not to scale. It's okay, Doc.
- The train leaves the station at 8:00 Monday morning.
- What does this mean? Point of no return.
- Here you go, Marty. Connect that to the positive terminal.
- All right, Marty, you all set?
- Pull up to the DeLorean! Pushing the DeLorean up to 88 miles per hour!
[...]