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2008 Toyota Yaris S Sedan - Highway MPG Testing:

43.2 MPG - windows down, cruise control off, A/C off
43.8 MPG - windows down, cruise control on, A/C off
46.2 MPG - windows up, cruise control on, A/C on 50%
49.7 MPG - windows up, cruise control off, A/C off
(Specifics: 40 mile interstate loops, cruise control set at 66 MPH, 60-70 MPH range with cruise off.)

Looking at these numbers makes me wonder: Is there some break-even speed where its the same whether u drive with windows down-A/C off or windows up - A/C on?
And what about windows partially open?

From what I have seen, there is a break point and it depends on a few factors:

What vehicle (mostly the CD factor and how much the windows change it, but also how well it can take the extra engine load)
How far down are the windows
How much AC

For slow driving it's usually best to open the windows. Air resistance isn't as much of a factor then.

I'd guess the tipping point would be somewhere in the 45-55 mph range for a sedan. I haven't found any difference with the windows cracked an inch or two.

Of course, windows up and ac off is the best combination.

2008 Toyota Yaris S Sedan - Highway MPG Testing:

43.2 MPG - windows down, cruise control off, A/C off
43.8 MPG - windows down, cruise control on, A/C off
46.2 MPG - windows up, cruise control on, A/C on 50%
49.7 MPG - windows up, cruise control off, A/C off
(Specifics: 40 mile interstate loops, cruise control set at 66 MPH, 60-70 MPH range with cruise off.)

Looking at these numbers makes me wonder: Is there some break-even speed where its the same whether u drive with windows down-A/C off or windows up - A/C on?
And what about windows partially open?

On a side note, your numbers are great for showing people how to take their Yaris into the 50mpg zone.

strange, both best and worst are with cruise control off. That doesnt make it any clearer now, does it?

It just shows how the driver makes all the difference, both positive and negative.

And implies that windows down hurts mpg.

Windows down an inch or two is probably the best as it doesn't make nearly the same impact on aerodynamics. I also think air conditioning would work well when used in combination with DWL - Use it on downhills and flats, but turn it off when going uphill. Alternatively use it during P&G only during the pulse. I would imagine the break even point around what others have said ~ 45-55mph for most vehicles. On my car having the windows down doesn't seem to affect my glides below 45mph. However I can definitely tell it takes more gas when the windows are down at 60 mph+