shape is as important as cross section
a normal bicyclist has a Cd of ~1 but a bicyclist in a high end fairing can have a Cd of .1
usually we want that as drag reduction - but you could use it as size multipliers
Thus a good fairing allows you to enclose 10 times more space
or a combination
a fairing that is twice as big but drops your Cd to 1/4 of it's original still drops drag by 50%
same with wheel covers (fairing for your wheels)
In my truck's case the convex almost perfectly aligns with the bulge in the tire
it is a very smooth combo (see pic#2 in first post)
but it all depends on your vehicles aerodynamic configuration
so your milage may vary
