I got my Scangauge a few weeks ago but didn't have time to install it before I left on vacation.
As soon as I got back I quickly put it to good use by reading and resetting the "check engine" in my Daughter's car (which I own and pay expenses on). It ended up not being anything serious and I was able to get it to pass emissions inspection without spending any money on diagnostics or repair.
Finally a few days ago I set it up in my Mazda 6. It's in the middle of a tank, and I haven't tried to calibrate it yet, so I'm just watching to see what happens as conditions change and not counting on the numbers to be precise yet. Very interesting none the less.
I got to verify things I've read here, such as the amazing FE while gliding in neutral with the engine at idle, lower FE while warming up etc.
It appears that I get about 38 to 42 mpg on level stretches with cruise set at 59. It will drop about 5 or 6 mpg when I turn the A/C on.
I haven't noticed a big difference in drafting situations. When a truck comes by there is some fluctuation, but when I've tried to stay around them I don't see a big improvement. I'll keep working on that and see if I can find a safe "sweet spot" that is worth staying in.
Yesterday though, I saw the effect the wind can have, which surprised me a bit. There were storms blowing into the area, and I had a brisk tail-wind on the way to work. On the flat stretches, again at 59 mph, I was now getting 49 mpg and sometimes into the low 50s!
Later, going the other way, the winds weren't as strong, but I saw FE of about 36-38 mpg.
Very interesting. I can see why some people get interested in aero mods.
I should finally have some new tanks to add soon. I haven't missed any, it's just that I wasn't driving my cars for a week, and at home I split time between the two cars so it takes longer to get through a tank on either one. It does keep hard miles off the new car though, which is nice. So far it looks like the current tank in the 6 will be a little better than last time (going for 40) and the Legacy TBD. After I fill up I'll reset the Scangauge, calibrate, and have a better idea how it's going as I'm going, so to speak.