To get your cars max potential u need to max the car all out! If u looking to beat the campaign as fast as possible tune the weight and hp boost on pro,and everything else on stage 1! No need for supercharger or turbo to beat campaign!
To the off topic what cars to choose in campaign: dclass-dart , cclass-frs , bclass-evo8 ,aclass-srt8,sclass-srt gts, rclass-rx7. But there are better cars to buy from showroom for each class,except for the evo,what is the best in bclass!!