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Newbie here, although I've read the site for years, never had a reason to post until now. I have a 95 vert that is idling high and experiencing a pretty significant drop in fuel efficiency (around 22mpg on freeway when it should be getting around 28-29). Took it to my dealer and they swear its idling within specs. I've owned these cars for years and know it shouldn's be turning 2400 @ 70mph.
I had a SES light come on and computer is showing a bad o2 sensor. It is being replaced today but the SA told me that a bad o2 sensor wouldn't be causing a high idle and also wouldn't cause enough of a drop in fuel efficiency to even notice.
I'm not sure what rpm your motor was turning @ 70mph before, but if it is higher now, your converter may not be locking up during cruise conditions which will greatly affect fuel economy.