After you completed all normal PR Challenges to 3 Stars you have some Special Chellenges left.
You can check them in Brick Challenges Screen, the red bricks in Bottom Row
One has a Achievement
I Used Mosler MT900S X999 and did the first jump from Airport side
The others are:
- Lego Car, 3 Stars within 60sec Area 7052 Speedtrap & Brickchester Speedtrap
I used Senna, start with Area 7052 - easy 160km/h, then streets to city and get over 321,9km/h also easy
- Extreme Tracktoy, 3 Stars within 45sec Litte Brickworth Speedzone & Brickchester Speedzone
I used my X999 Mosler MT900S
- Modern Muscle, 3 Stars within 60sec Waldkurven Drift and Hügelkuppe Drift
I used my '18 Mustang GT - S1 900 AWD, deaktivate TCR and STM in Settings, start from top of the map, in Forrest Zone you have to beat 100.000 and in Hills 75.000 Points
- Rallymonster, 3 Stars within 60sec Wüstenflucht-Wegbereiter and Festivaljagt-Wegbereiter
I used Hoonigan RS200 S2 998, (don't forget to reactivate TCR and STM)
this car you could get: Season5 Event, S6 Ranked, S12-4 ForzathonShop
If you don't have this one, try an other Rallymonster
You can aktivate the Bricks, to track your progress in the Challenge of the PR Stunts