Welcome Jorge. I have done “stealthy” aerodynamic mods to my Golf TDI and improved my mpg and coasting distances significantly. From outside the car you would only see the deleted passenger side mirror (and the folded driver’s on the lightly traveled highway). I blocked the upper grill with black pipe insulation stuffed between the grill bars. There seems to be plenty of airflow through the bottom grill. I have a Panzer plate that starts a bumper to bumper belly pan. I installed Votex side skirts painted to body code and used plastic rain gutter halves for inner and outer side skirts like the Aerocivic. I deleted the muffler mostly to make the rear diffuser angle better. I used rain gutter to make strakes that line up with the rear spring perches to complete the rear diffuser. I have small spats ahead of the front and rear tires. Please check out my albums and fuel log for more details.
https://ecomodder.com/forum/member-c...n+install.html
VW did a pretty good job on the outside but the underside is a mess. My coasting distances are much, much better than before the underbody aero mods. Good luck with yours.