I did some research in my garage and found out some interesting things:
1. Doesn't the online racing count towards the mileage? I have a lot of 0 km cars that I know for sure that I have used a lot online. Which is a shame, because nowadays I almost only do online racing...
2. Seasonal events counts though. Just bought a McLaren for the British lightweights and yep, the kilometers are counting.
3. Speaking of the McLaren Mp4, there's a pretty nice detail on the instrument panel, to the left of the steering wheel there's a display where you actually can see the mileage counting. It's the first car in GT5 where I have seen it (not that I've been looking), pretty cool.