bandit, mine is a Golf TDI, not a Beetle but I started a thread that you may find useful.
https://ecomodder.com/forum/showthre...sel-38306.html
As freebeard has suggested, the drag at the back of the Beetle is terrible. The curved roof creates strong counter-rotating vortexes leading to high drag. The wing may help disrupt those vortexes and allow the air to come together more smoothly at the rear of the car.