2D Ground effect for car+airfoil configuration
 

Dear all,
I'm working on the evaluation of the 2d aerodynamics of an ahmed body profile (serving as a car profile) with a detached airfoil generating downforce. I'd like to run Rans simulations with SA turbulent model. I'm having a hard time to obtain a stable and converging simulation after trying all the options I found in this forum related to ground effect and moving walls.
My mesh consist of a rectangular domain in which (wrt to the car) front, back and top sides are called FARFIELD, the bottom side that i want as a moving wall is called ROAD. inside the domain there are the CAR (ahmed body) and the WING (the airfoil).

I obtained this cfg file from a working simulation that doesn't have any moving boundary and these are the modification I added to it, but probably something is wrong within them.

% -------------------- BOUNDARY CONDITION DEFINITION ------------------
MARKER_HEATFLUX= ( WING, 0.0, CAR, 0.0, ROAD, 0.0)
MARKER_FAR= ( FARFIELD)
MARKER_PLOTTING= ( WING, CAR, ROAD)
MARKER_MONITORING= ( WING, CAR )
% ----------------------- DYNAMIC MESH DEFINITION ----------------------

MARKER_MOVING= ( ROAD )
SURFACE_MOVEMENT=(MOVING_WALL)
SURFACE_MOTION_ORIGIN= (0, 0, 0)
SURFACE_TRANSLATION_RATE= (55.4526, 0.0, 0.0)

Mach number at the back of the car increases drastically, especially near the FARFIELD boundary.
Is there someone with a similar case that can share a config file or show me where i'm mistaken?
I will attach the entire config file.
Thank you in advance for the help and sorry for the long post.