Do a search on the forum, someone posted a very nice setup that you could use as a starting point.
Each race then requieres a specific gear ratio, and the bumpy tracks need some suspension tweaks.
Then it's all about what electronic assist you use. usually the more you put, the slower you go.
I golded the championship recently, using a Dual Shock 3.
Did 1st on Fuji, 1st on Indy, 4th on Monaco, 1st on the nurgurgring, 1st on Monza under the rain. At this point my lead was big enough to abandon the Suzuka and still win the championship.
Each track got its section where you can gain some precious seconds :
Fuji, you can get seconds in the fast curves.
Indy is not a very technical track
Monaco is a PITA, but there's a curves where most car tend to do 360...
Nurburgring, you can cut the last chicane full speed. And every single GT do at least a 360 at each lap...
Monza, cut the chicane on the grass + AI doesnt know how to drive under rain
Suzuka, cut last chicane + good paces in the S + AI under rain
Each win is about 30k exp.
