The DeLorean that appears in the Back to the Future movies was actually more custom-built than you are perhaps giving it credit for. Those with an eagle eye would notice that the speedometer that appears on the car in the movies goes up to 95mph, and at first you would think that this was nothing unusual or special. However, that would in itself be wrong.
It turns out that the original DeLorean had a maximum speedometer reading of 85mph as a direct result of a mandate that was passed down by President Jimmy Carter. The problem was that it did not fit into what they wanted for the car for the movie, so they had to then go to the extent of producing a fake speedometer that worked and met their needs.
So, you really cannot believe anything that you see in the movies not even something that is apparently as small and insignificant as that.