VW Golf windows
 

Source: Energy Efficient Transportation for Florida

That really pretty good with AC on only to lose 12%. On our Honda van it's 20%. With the windows, was it all windows open all the way or just the front windows. I couldn't tell from the article.

It doesn't say.

104f/40c
 

Honestly, I'm an absolute wuss when it comes to the heat. I'd perform some serious other mods before giving up my beloved cool air. Zpilito can probably help me here, but 104F/40C with high humidity is downright miserable. I do appreciate the science behind the open window theory (to which I subscribe). It really depends on the spare engine load and the compressor resistance, which varies with every vehicle. In the mean-time, I'm trying to use the re-circ button all the time to reduce the need to compress as often.

I was thinking that when the A/C days go away, we should see an increase in FE for those of us that will have used it. Since we're getting Ethanol instead of MTBE all year, there shouldn't be much change in energy-per-combustion. The true test will be whose vehicle likes an HAI or not :rolleyes: (and also cold starts - of which some have an EBH ;))

RH77

Only 2.5% change in drag with windows down? I don't think so, that's like.... really really tiny. An increase in wind speed of 2mph can increase fluid drag by 10%, which is why it's kinda hard to sniff out aero improvements unless you take the average over thousands of miles. Or have a device that can show you the instantaneous energy required at certain points, which you can then compare many times.

Out of curiosity, what size of change would you have expected to see?

I would've expected something larger since it seems like the windows are larger than the wheel wells, and would induce more drag because the area they're near would usually have minimal seperation/turbulence, unlike the underbody. So... I went outside and measured the windows and rear wheel wells on my two door mk1 golf, apparently my mind is playing tricks on me, because they have about the same area! ;)
In terms of calculating increases in FE wrt drag, since very slight changes in wind speed and direction can result in differences in energy that are much larger than 2.5% I wonder how accurate those tests can be? Specifically, a .82mph change in opposing wind speed at 67mph will lead to a 2.5% increase in energy required to overcome fluid drag. I can't think of any place that's not subject to 1mph variations in wind speed. Course I'm no meteorologist... ;)

I think that's reasonable. I base this mainly on the fact that I see no noticable instantaneous difference when I roll up/down my windows, and it really doesn't get all that loud/windy in my car. I've been thinking about pulling my a/c while the car is ripped up, but Iono if I should, I'm sure the folks will get super pissed off, though it's not like it's every worked anyway. Mehbe towards the winter when I'm home I'll do it.