Well, I can tell you that in AZ, where it is frequently over 100 degrees in summer...basically May-Sept, the ECO mode definately doesn't cool the car off as well. We took a 100 mile trip on the freeway last week, with outside temps at 100, and with ECO mode, it wasn't comfortable in the car. I switch it on and off occasionally to maintain a comfortable temp.

It is my understanding from an earlier thread that the ECO mode will not cool the car below 77 degress, regardless of the set temp...(which causes the fan speed to constantly incease...on auto setting...to try to keep up). With 100 temp outside and a huge heat load from the bright sun in the desert, the ECO mode is barely tolerable for us. Once the temps get back to the low 90s, ECO seems to work fine, or if it is cloudy, or at night (yes it can still be near 100 with the sun down)