I don't think there is a native function for change in rotation. You'd have to subtract the current rotation by the last rotation.
//not sure what language you're using so here's an example for C#. don't actually use the wait(1000) in a mod
Vector3 oldRotation = Game.Player.Character.CurrentVehicle.Rotation;
Vector3 newRotation = Game.Player.Character.CurrentVehicle.Rotation;
Vector3 changeInRotation = newRotation - rotation;
Vehicle speed is:
float speed = Game.Player.Character.CurrentVehicle.Speed //c#
float speed = ENTIY::GET_ENTITY_SPEED(PED::GET_VEHICLE_PED_IS_IN(PLAYER::PLAYER_PED_ID(), false)); //c++
Turning radius is (.NET only):
Only documentation I look at is the Native DB. I also look at the source code of Script Hook V .NET.