How would car data make any difference

Criminal's lawyer: "Your honor, clearly my client was being chased viciously by that officer, here's the car data saying it was going 65mph"

Judge: "Yes, and as we just saw in the video footage from three news helicopters, your client was travelling even faster than the police chasing him, weaving in and out of traffic, before losing control when his tire blew".

Engine run time, speed, location, engine temps, impact, hard breaking, hard acceleration, this is the type of data that is being collected.

Also I would imagine fleet vehicles, like police cars and taxis, would not include most of this as it increases the costs and complexity of repairs. Fleet vehicles are generally designed to use the same major components as multiple other vehicles/generations and be fairly no-frills features to be the easiest to work on. Major police departments (large cities, state police) generally have their own garages that do all of the work.

If police cars have the same data-collection systems as consumer cars, all phoning home to the GM/Ford mothership, then those police departments won't be able to "lose" data. A company the size of Ford isn't going to cooperate with some puny little police department in Arkansas or wherever in illegally destroying evidence.