Fuel consumption with AC on?

I'm pretty curious about the fuel penalty for AC use; anyone with a scan guage II or a car with a 'puter can chime in here; I am interested in Miata's and other cars as well...

Say your cruising along at 65mph with ac off, then turn it on; how much does the mpg's drop? Most of those fancy car computers give you instant feedback...

Also, what is the fuel consumption, running on a flat road, at 60? Then 65? Then 70?

I guess I have been a bit obsessed with fuel economy at times.

Anyone have a Scan Guage II they want to sell (or even rent for a period?)

On average my 2003 RAM 3500 CTD gets about 1.5 MPG less with the AC on while not towing. Oddly enough, while towing it does not seem to make a difference.

What about the old argument concerning drag.

Windows/top down, no AC causes more resistance on the moving car, correct? Windows/top up, less resistance. I can't imagine it making a big difference either way in a Miata.

No scangauge here, but if I run a full tank without using the A/C at all, I get between 29 and 30 mpg. If I run the A/C half the time, (for example, A/C driving to work, no A/C driving home,) I get about 28 mpg for the tank, so I'm guessing the penalty is about 2.5 mpg or so for my '99 miata.