I did the math..
 

Maybe I'll get blasted for this but I actually found this humorous. In fact it fortifies the reason I push myself to get better and better mpg tanks, always trying to beat my personal best. It is almost a hobby.

Ok, I added an air-dam and lower grill block using garden edging. I already had it lying around form the last car I added it to but lets say I spent $20 for the edging and zip ties. How long would it take to show a positive return on investment?
Lets go with 45 mpg without it and gain 1/2 mpg with the air-dam for 45.5mpg.
1000 miles driven / 45 mpg = 22.222 gallons used
1000 miles driven / 45.5mpg=21.978 gallons used
22.222 - 21.978 = 0.244 gallons saved
$3.85/gallon x .244 gallons = 94 CENTS saved per 1000 miles driven. Wow, awesome.
After 20,000 miles I have almost got back my initial investment.
Even if it brings a 2 mpg gain that would still only save less than one gallon per 1000 miles driven, or about $55 per year (@15,000 miles).

Was it worth it? You bet. Every new mileage increase adds up and is fun for me. As an added bonus, the goofy looking air-dam gets people talking and thinking about their own driving habits.

Wont get blasted or flamed for doing anything like that to a vibe around here.

Now if you put lawn edging on your CRX and went to a CRX forum you would get flamed because your lawn edging isn't a $400 uglyified fake carbon fiber aftermarket ground effects kit sold as "JDM" that required bondo to fill all the gaps in.

I'm wouldn't flame you, i do the math all of the time to see when a particular mod will pay itself back.

it's all part of being an ecomodder :)

That is too funny. The "left over" lawn edging I had lying around is from when I bought it and DID install it on my CRX (pictured in avatar).
I meant blasted for poo-pooing the actual miniscule dollar savings per mod.
Thanks guys. Just having fun and trying to put up good numbers on the fuel logs.

I am simultaneously blasting, flaming, and trolling you--in my head! :D

Just kidding. I would not have any idea what to say.

People ask me about this stuff, which is a very good question, but as has been mentioned before, is this really worse than spending much more money trying to make my car faster? The difference here is that we try to make sure that our modifications actually work and if they do not, we talk about it, and probably remove the modification.

right.
And, the largest gains in mpg are learned (driving habits, dfco, eoc, p&g, etc.) not bought. Free!
Another good gain is really really really cheap.. high psi in my tires. If you wanted, I suppose you could calculate the electricity I paid for to run my air compressor to fill the tires (tyres :) ).

See kids, math CAN be fun!

I often have the same conversation with others about when things will pay for themselves and I think that is why so many on this site are always looking for free materials and ways to do things cheaper. Prime example is pizza pans vs. moon discs :)

$55 could fill up your tank once a year though and for me one gallon every 1000 miles saved means that @ 15000 miles I've saved more than one tank of gas. So it's not something to sneeze at.

Wait, you mean these mods can pay for themselves? I need to get better at that.

I'm at less than a hundred bucks for all of my mods, including the sweet paint job, and 70 bucks went for adding the kammback alone.

Even though I'm pretty sure that kammback won't ever pay for itself in mpg savings during the life of the car, I did get a "wow, that looks better" from my wife (who carpools with me in that beast) after I took off the detached wing and added the kammback- - -so technically it's already paid for itself