How much does a belly pan or front air dam decrease drag?
 

I was thinking about making a belly pan for my 05 Civic to reduce aerodynamic drag. Then I read on this forum that I can get 75% of the aerodynamic benefit of a belly pan just by making a front air dam. If that is true, I would much rather save the time, money, effort and weight and just make a front air dam. How much of a difference does a front air dam make? I am considering making one, but I am not really a fan of the look. I would be OK with it though if I could gain at least 0.5 MPG on the highway. Is a gain of 0.5 MPG or more likely to happen? Thanks for the help, I am new to Ecomodder.

Almost as much as an air dam.
The front under the car is so aerodynamically poor it's best just to block off the air flow.

But would an air dam block the airflow under the car? To me it would seem like the air would just go through the gap at the air dam and just go under the car anyway.

Yeah it's one of those counter intuitive things that has been proven time and time again in the wind tunnel.
That's why the belly pan improves aero. Because all the stuff hanging down, pockets and gaps causes a ton of drag.

Very interesting, thank you. How much of a difference does a front air dam make? I am considering making one, but I am not really a fan of the look. I would be OK with it though if I could gain at least 0.5 MPG on the highway. Is a gain of 0.5 MPG or more likely to happen?

The general rule is airdam no lower than the lowest part of the undercarriage. Frontal area isn't affected.

0.5mpg starting from 20 or 60mpg? Probably doable at 30-40mpg.

The belly pan gets increasingly important toward the rear. Under the engine compartment you should be aware of cooling air flow. At least a partial pan behind the rear axle will set things up for a diffuser, which is at least as important as the air dam.

difference
 

I GOOGLED images of the '05 CIVIC.It looks like Honda may have already optimized the airdam on the car.
There is a chance that lowering the airdam down to around 3-inches of ground clearance might cut drag an additional 1% but it would strike every driveway ramp,curb,and parking lot barrier unless you were very vigilant in your driving.
$1,000,000 concept cars and supercars get around this with active aero and adjustable-height suspension systems.
I'd have you look under the next Tesla you can find and inspect how Musk's team engineered that belly.You'd be looking at the industry standard for a low drag belly and diffuser.
I can't tell you what difference in Cd it would make.But if you're interested in aft-body streamlining later on,the belly would insure really good flow back there,which in mandatory for boat-tailing to function to it's highest potential.

Welcome Eco, I have used airdams on many vehicles over the years and they do help mpg at highway speeds. As Aerohead stated they are very easy to strike on all kinds of objects even if you think you are being careful. Several of my airdams were commercially made including the GLH factory airdam I installed on my Dodge Omni. I can tell you I have broken every one. I use a full belly pan now and it is holding up very well.

I found on my Rondo that a full air dam hurt my mileage but a partial on both front corners and installing a belly pan helped. Different cars and different air dams are going to perform differently. You may have to do some experimenting with cheap materials first. I am a fan of belly pans. They are more work, but are more effective IMO. The other advantage of the pan is raising under hood temps and reducing warm up time in colder weather. In my mind it's a win win. Only downside I've found is mine increase cabin noise a bit.

What about an active airdam that lowers itself one above a certain speed?

Would an angled airdam have any negative effects? Something like a hinged panel that is attached on front and lowers itself.

Or the inverse, atached at the back side and is pulled up with a small servo motor via rope. The wind would swing it open when the speed goes up, the servo would pull it up when not needed.