From what is said in the manual, I believe that the changes in vehicle performance really only apply to city driving. Highway performance might change if you are doing a great deal of downshifting at highway speeds.
ECO mode does apply to all speeds. I've found that I really like it on highway driving. I personally dislike downshifts at a steady speed. So I'd like a car to downshift only if really really needed. The ECO mode seems to help the transmission keep it's gear longer and remove some un-needed downshifts.
Eco mode basically translates to lower idle speeds and earlier shifts, with an emphasis to getting in a higher gear faster. On the highway it doesn't translate to much. Most people say it adds about 1 MPG.
I'd say the ECO mode is either on or off at any speed or driving condition. The vehicle should shake a bit in ECO mode in the city due to lower idle speeds. I've found I like it turned on all the time now, as the shift points seem to match my driving style a little better than with it off.
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