A recent example of drag degradation due to a 'void' would be the 2013 Bugatti Veyron 16.4 Grand Sport Vitesse.
When the roof panel is removed the top speed falls from 255 mph,to 233 mph.
A 22 mph drop.Or,a 9.44 % reduction in top speed due to the extra drag created by the void.
Using Wolf Heinrich Hucho's drag/top speed relationship loosely,this 9.44 % change in top speed would indicate an approximate 28.3 % drag increase.
