You really think people who spend $50K and up on a car are that worried about gas prices?
The idea behind the data recorders (which are actually already in many cars) or 'black box' is to have a data record of everything that was happening during the last few seconds before a major 'incident'. Eg, airbag deployment, impact detection, etc. This way, the person goes to court to sue their car manufacturer for making a car that would 'just crash', then the manufacturer pulls up the data and asks if the plaintiff suspects that the fact they were doing 95mph might have contributed to the problem.
I'm on the fence here... as long as this data is not remote queryable (eg, must be physically retrieved), only records the minimum length of time needed (eg, a few seconds, not hours) and the data is used in an NTSB type function (to determine the cause of fault, not to make a bunch of lawyers rich), then it sounds ok.... but what are the odds that's what'll appear?
What we'll REALLY get within the next 10 years might be boxes that track speed, location, etc for the past hour and wirelessly upload it on demand. So Johnny Law might set up a speed trap where he doesn't actually have to clock you speeding, he just needs to ask your car if it has sped recently.