So what does an open window represent, in terms of change in Cd and fuel consumption?
I haven't measured it on my car (and can't because my cruise control is still AWOL), but someone else has tested this with a similar car, so we can maybe draw some conclusions:
See:
Windows down on VW Golf = 2.5% FE reduction
A similarly shaped car to mine, a VW Golf, was tested at 65 mph with both windows up vs. down.
Quote:
"We also found that closing windows at freeway speeds improved fuel efficiency by 2-3%."
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Let's average that and call it a 2.5% difference.
Based on the aero calculator, to achieve a 2.5% improvement in fuel economy at 65 mph with my car (roughly similar in shape & Cd as the VW GTI tested), Cd would have to improve by 0.01
Considering that VW data is for BOTH windows up/down, that suggests that the resolution of coastdown testing one open window on my car at 100-50 km/h is about half that, or Cd 0.005
Quality of the data set makes that debatable of course.