it all depends on what's under the car and what you're trying to achieve.
an airdam is generally not considdered an optimal solution. it pushes the air asside, and most likely also below the car, and that all takes energy. however in some cases it can help. either because it prevents almost all air from getting under the car, like on racecars. or because it's big enough to shield something but not add signifficant frontal area... in wich case it's probably only a few inches high
you might want to look into underbody paneling instead, that might be more difficuly to make but also more effective.
if you do go for an airdam i'd say use it to shield something that would otherwise cause drag, like the engine and suspention components.
look underneath the car and try to see what hangs out below the line of the bumper and try to shield that.
again if you're looking for a real improvement and you have enough material available you might want to look at an underbody panel below the front bumper. afterall it's better to gently guide the air where you want it to go, than forcefully prevent it from going where you don't want it.
ironically enough, just when i'm thinking about an underbody fairings, i might get my hands on a rubber airdam extention form an opel corsa... might fit my car with a little work, so perhaps i might try that as well.
2-3mm or 1/8" aluminum would be more than fine. I would use coraplast or cardboard first to get the correct setup before you invest in Al.
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Don't worry, it took me a long time to figure it out on my own. Corogated plastic = Coroplast
It refers to the construction style that normal cardboard boxes use, only done in a plastic material. Sign shops, craft stores, and places like the Home Depot may carry it, but don't be surprised if you get a funny look when you ask for it.
31 mpg city in my S60
Been seeing 37 on longer errands
Right now I like thin plywood best. Home Depot etc. sell 2.7 mm Lauan plywood. That's less than 1/8 inch. Curves when you want it to. Also sell 5.5 mm. if you want it stiffer. Plenty airflow so I'm not worried about rotting. Or you can prime with oil base house paint where needed.
Coroplast + duct tape = goes together as fast as you can plan and cut the material. BUT you'll be repairing duct tape problems weekly. Sometimes daily. And leaves a nasty mess after you pull off the tape.
For the coroplast thing I had this past summer I did better with sheet metal screws thru the plastic into little wood strips. Or into drywall corner bead as a cheap "angle iron" type stock. They make a flexible corner bead for people making arches.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.