rav -
The consensus is that adding to the frontal area hurts Cd. On a very old car with terrible Cd, they may actually help. This is a 1981 Peugeot Vera that was aeromodded :
They claimed a benefit :
If I were you, I would only add them to the front windows. I originally had 2, but now I only have one on the driver's side. I use it to delay/avoid A/C and to vent the cabin when it is parked.
CarloSW2