Aero question
 

So, I have my '67 Mustang Convertible. If I were to get some landscape edging (it's green, same colour as my car!) and use it as a front air dam, what would be your guesstimates be as to my gains?

From the factory, a '67 Mustang was probably around ~,45cD, probably more as mine's a convertible. But, mine's lowered 1" from the factory ride height.

If pics of my undercarriage would be of help, I can take some. Aerodynamically, it's pretty dirty.

It depends on what kind of driving you do, most of the time 1-2mpg.

My commute is 10-13 miles, the first 7 of which are on country roads with a high RR and a self-imposed speed of 40mph. After that, there's a signal with one mile of state highway @ around 50mph. Then, there's a large road with signals but speed resumes @ 40mph.

If I take a roadtrip (so far this year, that's been about 2500 miles), I limit my speed to 60mph since I'm without overdrive.

There have been some good looking air dams on this site done with garden edging. It's a relatively low cost project so as long as it gives some benefit I'd say it's a good thing.

I suggest looking in the Aero section and see if you can find some of them. Mostly bigger vehicles if I recall, but there's no reason it can't look good on a Mustang.