[elhigh]

I thought the general consensus here was that an airdam was good for 2-5%; even the modest 2% improvement would be good for about another mile per gallon. So while you're being conservative with your numbers, I daresay you're being TOO conservative.

Don't forget the savings snowball: if you don't need as much power to push your car down the road, then you're running the engine at a lighter load, less stress on moving parts, longer service life etc.

In terms of dollars per year, I think making the car go an extra few years is a savings that is both very real and very difficult to quantify.