also the type of person who is going to be using the car and putting crappy fuel into it or not servicing it or anything else that could lead it to have problems
the chip just optimising the fuelling so you get more power and more mpg (perhaps not necessarily at the same time though) people who have had them fitted have come back with the general "used lots more juice in the first few weeks and then settled down to give much better results" the argument used here is that, if you had more power at your disposal you would be using it more until you got into the habit of not needing to use it anymore so you used more fuel when you where hooning it round the local shopping centre car park but once you got used to the power you went back to normal driving and saw the better results in fuel economy