rkcarguy

I dissagree. Possibly with more stop and go the hatch will see better mpg's due to weight loss, but I have owned several EG's and noticed slightly better freeway MPG than the hatchbacks. One thing that helps though is every EG I've owned always gets dropped and a front lip, which helps.
IIRC the Si and Vx had slightly lower CD(.01) than the rest of the models probably due to the lip.

ESmooth

Agreed, lowering the height of the car and controlling the airflow beneath it will make a differnece. Going with a flat bottom is actually better than a lip since you arent increasing frontal area but either way can help, assuming there are no canards or excessive scoopery going on with the lip.

brucepick

Hatchbacks and aero...

It's possible for a coupe or a hatch to have a lower .cd than it's sister sedan but not necessarily.
Look up Kamm Back in wikipedia.
And/or go to http://ecomodder.com/forum/aerodynamics.html and start reading the stickies at the top of the Aerodynamics forum.

Here's some of the how and why.

One goal is to make the air flow follow the car's contour. Or more accurately, design the contour so that the air can follow it. A curved or angled surface that the air can follow at 40 mph might no longer work at faster speeds.

Another goal is to create the smallest possible wake. Someone posted re. fish, birds, and may have also mentioned teardrops. Those are basically the ideal shape, with the tapered end of the teardrop at the rear. The air/water can follow the gentle curve/slope as it tapers, and the airflow over top, bottom and sides joins back together where the tail tapers together.

The opposite of a small tapered back would be a full-size square back, like an 18-wheeler trailer. It sucks a huge vacuum behind it. The vehicle's engine is working to produce the energy to create that vacuum. Wasted effort. It shows up as a higher .cd and reduced mpg.

You can chop off that teardrop tail at any point and that works very well. Not as well as continuing to a fine tip but better than a full width, full height, back end.

The benefit that a coupe has, is that the rear seats don't usually have the same headroom as the fronts. The roof can start sloping down a bit earlier so the angle is more gentle. That way the air flow can follow the body at a higher speed, or follow it better assuming the same speed. The Civic couple does this, in comparison to the 4-door. I visually compared the '96-00 body styles and it's true, sedan vs. coupe.

A hatchback can have a gently sloping rear that can also do this job. However, many hatches keep a lot of height, and the Civic hatch is an example. Not that good for aero, compared to a back that tapers to a smaller cross section, but handy for carrying stuff. And probably better aero than compared vs. SUVs and wagons, pickups, etc.

kulo

will having the sunroof open affect mpg?

or get a moon roof visor

like wind deflectors

brucepick

Generally yes - for any of those. The smooth path over the roof is interrupted when the air moves in or out of the open sunroof. Of course if you don't run the a/c because the roof is open, you probably come out ahead due to the high load the a/c would create.

Visors and wind deflectors - same idea. What would be a smooth air flow path is now a path with bumps and detours. Your engine is spending energy moving air around.