Can't compete with the Roadster Credits but I can get an easy +2mil at Eifel
Since the 2.10 update a Callaway with engine +3 Weight +3 and fully tuned without aero the PP is 516 with 752 hp
With 200% login bonus
you can win fairly easily taking the lead on lap 5 and earn A Performance Difference Adjustment 1,451,008 -- Total Credits earned 1 race 2,148,608
Use Praiano's Tune -- I tweek the transmission for my preference