Originally Posted by Scahpe
When the ambient temperature is above 100f, the A/C has to work very hard to cool it off. Yes a compressor works at normal idle, but not as efficiently as slightly higher than idle (example 2000rpm), especially when the inside of the vehicle can be well over 130f!
Of course, protecting the battery is Ford's #1 concern on this system (They don't want to warranty these any more than they absolutely have to.. and I don't really blame them) so they want to cool it off as quickly as possible.. even if fuel economy takes a hit in the process.
It is a pretty serious hit. My mileage was down to 24.5 while the engine was at "high RPM". The thing I really don't understand is why it happened at highway speed. At highway speed the compressor is always on, and 1700 RPM (on the downhill) should be enough to cool. Not to mention the uphills at 2600 or so...
I have managed to get my MPG back up to 31.6 these past two days...