In the scene where Michael J. Fox's character has gone back to the 1950's, when his mother and father were still young, and is trying to get the almanac back from the young Biff, watch the chase scene carefully. If you look at the part where Biff is about to run Michael over and ends up crashing into a manure truck, you see him put his car into gear with an automatic gearshift. I wasn't aware automatic transmissions were around, or if so, common among the youth during that time. Hmm...