Aerodynamics should benefit BOTH Top Speed and MPG, right?

[Cd]

AND your car will make you look sexy.
AND attract women.

Win, win, win, win situation !

Winning !

[kach22i]

To answer the original question, to focus only on aerodynamics (low Cd) neglects safety at high speed.

Things like side winds can ruin your day (kill you), so some attention to center of mass and center of area and any cures or treatments for danger should be dressed despite the fact tails, fins and the like will add drag.

Down-force or lack of lift at speed is important and may involve some drag generating liabilities if done after the fact.

The things we ecomodders are apt to do with our existing vehicles typically do not include venturing into any high limit danger zones.

In fact some of these guys are more likely to drive too slow on the highway for those crazies wanting to do +20 mph over the speed limit in their brick-like SUV's.

EDIT:
Nice posts on that first page.

Here is a thread from about four years ago which hits this topic hard.

Example.........................
http://ecomodder.com/forum/showthrea...s-11183-7.html

Originally Posted by aerohead

If you were to spin a car like a top on a slippery surface like black ice,you'd notice that it appeared to revolve around a 'pole' as Earth does.
This 'polar' position would represent the longitudinal position of the car's center of gravity(CG) and the car's polar moment of inertia would pivot about this point.
If you were to balance a car on it's tail or nose,the vertical line passing the the longitudinal 'center' would define the car's actual C.G..
If you were to also direct a strong blast of air from the side of a car when on a turntable,when the car was so positioned that it neither spun from the nose,nor from the rear,but 'balanced',with no yawing at all,the position of the turntable's pivot shaft center would define the longitudinal location of the cars' Center of Pressure ( CP).
For aerodynamic stability the CP is kept behind the CG,then if a strong gust is encountered,the car will simply weather vane itself into the wind,and thereby cancel any yawing moment.

[niky]

If we're talking the OP's Honda Civic, those front-drive commuter cars are typically drag limited to around 110-120 mph... maybe if it's an auto, you've got a 117 mph speed limiter, but typically, newer manuals will hit top speed quite a ways before redline.

At those speeds, the car is still relatively stable in side winds... unless we're talking extremely windy weather.

[freebeard]

Originally Posted by kach22i

Except ....that behavior, yawing upwind, describes the behavior of Bucky Fuller's Dymaxion. Most cars veer downwind. I think the force applied through the center of pressure is distributed through the center of gravity to the tire contact patches. There it acts like wind in a sail, pressing the tire against the limit of it's slip angle like a keel in the water.