Yes, realistically the near ideal shape is a teardrop that is smooth. Placing roughness (and movement of the fabric) can cause the airflow to separate off the surface too soon. The airflow should stay on as long as possible then separate with some level of turbulence. Although this is a fairly short top compared to a teardrop shape.

 

I have experienced the same thing with my hardtop, but for me it's more like 1.5 mpg, I use an app on my phone to record all the mileage and mpg, it becomes a lot noticeable when you commute at 80+ mph. You start to hear the softop start flapping the faster you go.

Sent from my Samsung Fascinate

 

So how many years before the HT pays for itself?