Are you talking about in general or on a certain course and race? Well for me, I don't have any problems usually, but the key is to look at it as not a race, because GT6 AI isn't that much trouble and is bad anyways.
Its more or less a time trial with 6-12 fast moving traffic cones lol.
The AI is really bad at corners, and follow the driving line, but they do it VERY VERY SLOWLY, so what I would recommend is say your on a track like Mt. Panorama, stay behind and try to keep up on the straights and always draft as much as you can if you are using an underpowered car, and overtake on the corners, as 1000% of the time, you can outbrake and outturn and outmaneuver the AI in the corners, you can jump up a good 3-4 spots if you do it well.
PD pretty much starts you off in last place, cause if they put you in the middle, you'd win by too much. So just overtake in the corners and draft on the straights, once you get to first, the AI shrugs off and pretty much gives up, you can start to pile up the seconds ahead.
