Ok, the traction and driving up hill has been fixed. Somebody on Youtube helped me out. Apparantly my collision mesh was too low in poly, so now I have kept the same amount of polygons for both the road and the collision mesh.
Still trying to figure out why the road seems so narrow in-game, while in Blender I can place 5 cars next to each other. In-game it's like I have just a bit of room to steer.
And also don't know why I am not able to use gLTF for the road and collision shape in Godot, while gLTF works flawlessly for the car.