Funny, but here are a few things WARNING!! Spoilers!!
1. The rock that Jeff put in front of the back tired mysteriously disappears after "Toby" got "run over"
2. 250 cars. In real life, they would not all start. After sitting so long, the battery would go dead, and the fuel would also go bad. Most junkyards I've been to have very few cars that actually run, and if they did, they would not all be in good condition, and getting all those cars to the lot on time, without any breaking down, is a miracle. (I am not counting the one that was trashed by the police car)
3.The blocking of President Carter's address: How would they know what President Carter was going to say?? Jeff, as the "Sheriff" Kept saying "Did you hear what I said?" And Jimmy Carter says "I have heard you, with unmistakable clarity"
4.Pacemaker. That needs no explanation (ok, if that guy could build a pacemaker, why did he not works for medical suppliers??)
5. Luke's last ride. What are the chances that a car would follow a perfectly straight line without being guided?? Luke was dead, so he could not guide it into the power substation.
6. The pit. What was the pit used for prior to putting the Edsel with Luke in it?
That's all I can think of for now. I love the movie, but it's not 100% believable.