Corrected entry: When Marty leaves for the future at the beginning of Part II, Biff is in his red tracksuit about to wash their cars and Marty's black truck is in the garage. Then in Parts II and III, Marty and Doc make plenty of changes in the past and Doc staying in 1885 would greatly change things on Marty's return to 1985. But when Marty's returns to 1985 at the end of Part III, Biff is still in his red tracksuit about to wash the cars and the same black truck is still in the garage.
Gavin Jackson
4th Jul 2004
Correction: But bear in mind that Doc left 1985 at the same point as Marty, so any effect he's had on history up to that point will remain, despite eventually going back to 1885. The ravine name changes, but nothing else world-altering happened.