Virtual boat-tailing

[aerohead]

Originally Posted by Hypermiler1995

bump!
1976 BMW 3.5 Csl turbo
Attachment 16734
note the roof deflector

I suspect that the deflector was directing air to the rear spoiler.
The 1969 AMC Javelin had a roof spoiler mounted in this location.It had no effect at speeds up to 115-mph.
One of the Toyota Supras had one also,and it had no effect.
Mercedes-Benz had a 190 EVO with a non-aspirated roof deflector which also helped air to get to the rear wing.
NASA has experimented with Large Eddy Breakup Devices (LEBU) and they didn't work out for them very well.
Nowadays,VGs would be placed to do what these deflectors are doing.