Porsche 911 1974 undrivable cause oversteering (in BeamNG)

[titou45150]

Hi,
I created with Automation a porsche 911(911) 1974.
I had quitly the same characteristics (engine position, power, etc...) that the real car of the same period.
I mount tires : 600mm / 15 inches
Front/Rear = 185/80VR15
Car designer inform me that my car is suffering from oversteering, which the case once my car has been imported in BeamNG and when I drive it ... it becomes totally undrivable when it speed exceeds about 175km/h...
The real car could reach 220km/h ... I hope user of the time didn't encounter the same problem of aversteering like me ...
If someone could help / teach me what is the problem and how can i fix it IT WOULD BE REALLY GREAT.
Thx by advance.
I love these coupling games Automation/BeamNG.... so much