where to start
 

I've been lurking here for a while and figure its about time i start trying out a couple of inexpensive changes. I'm driving a 94 civic sedan with 210,xxx on the odo. I coast in neutral as much as possible, average 65 on my 20mi commute. My tank to tank average this winter (michigan) has been 41 +- 1mpg.

I was thinking about starting with a couple sheets of coroplast from a sign shop ($25 per 4x8' sheet) and shaping an air dam with tire deflectors, under tray and maybe rear wheel fairings.

Which of the above will give me the best gains in mpg? i've read that the undertray is not as necessary with the air dam and partial radiator block.

for those with civics-- how are you attaching the undertray to the bottom of the vehicle? i'm leaving a gap on each side of the exhaust system to allow for cooling and prevent melting/fire. are you just using self tapping screws straight up into the floor of the car?

any general tips/suggestions?

Do you already have a partial grille block? That's where I'd start.

(Standard disclaimer applies: watch temps closely - cover progressively so you don't overheat. And realize you may have to remove blocking as the weather warms or if you do any driving with higher than usual loads, like high speed or climbing)

No grille block yet, but i figured i would just use 4 individual panels. 2 over radiator side and 2 over air-conditioning condenser which would be more permanent (ac inop). warmer months I'll remove panels as necessary. This is just my work car so its rare for me to have any passengers or any load other than myself.

Grille block has multiple advantages over other aero mods. Higher return on investment among the "small" individual mods.

You could also overinflate your tires. On my liberty i have the tires inflated to 40 psi (45 is the max i can infalte too)& I have my toyota camry at 35 psi.

the sign shop that i'm going to get the coroplast from sells a 4x4 for 15 or 4x8 for $25. regarding an increase in mpg, is it worth the extra money and time installing the air dam & undertray vs. just blocking radiator?

If you're looking for the highest gains, slow down from 65 avg to 55 avg. That will net you a bunch.

$25 for a 4x8 is pretty high. A sign shop buys from a plastic supplier, and they are marking it up quite a bit. Call some more sign shops and find out where they get their supplies, and go directly to the people who sell plastics and you'll get a better price.

bellypans almost allways" pan out" if the bottom of the car is dirty aerodynamicly speaking, as to how to mount it ,i drilled holes in the existing plastic under my car and used bolts and washers,search echo bellypan install, there are quite a few how to's on this site for bellypans, i only did my front and rear as the middle of my car is pretty smooth already. coroplast is cheaper at plastic sales stores . they sell lexan, acrylic styrene etc, you can get it for free if its election season,but its usally smaller peices and harder to work with,i vote for pans and grill block.

In my limited experience, if you have a front air dam where the overhang is slightly more then the height of the lowest hanging part behind it (oilpan, control arms, exhaust etc..), then you are all set and imho that is the best mpg gain you will ever see. As far as belly pan is concerned gains are almost negligible in comparison to a well designed air dam.

As a general rule of thumb for an airdam what would be a minimum ground clearance to avoid scraping?