[aerohead]

Personally,I've never noted a difference.I've kept records for over three-quarters of a million miles on different vehicles,and never saw it on the radar screen.I can state,categorically,that an air conditioning compressor will dig into mileage.Popular wisdom has it, that the loss from running the air conditioner,is compensated for by the loss you'd get from air drag associated with "windows down",so its a wash.Prudence would have it that we take everything on a case-specific basis.It's probably up to the individual,to ascertain whether or not the open glass makes a difference or not.Sorry,I guess thats not much of an answer.Good question though!