kiraelric:

You will get to a point in the game where you have to cross a barricade in the beginning of chapter 7 - there is also a side quest you can do to enhance your car in the same area - DO NOT DO THE SIDEQUEST - and stay on the plot. The Sidequest will allow you to fast travel back to Hammerhead to enhance said car, but once you leave that area of the plot, due to said barricade you WILL NOT be able to get back in to advance the plot. There is no patch for this, and it is not fixable, so unless you have a save file where you are inside the barricade or have yet to enter it, there is no way to return.

 This is my happy warning to you all from someone who has to replay chapter 5 / 6 of the game now because I just wanted a faster car.

DO NOT rely on the autosave files, they update too quickly. Make a hard save file, you update yourself. It will save you a headache just in case. 

@mastermod