Switching the OD 'off' means you do not shift up into that last notch in the tranny. In my 3.73/auto dropping into/out of OD yields about 650-850 rpm difference at a given mph.
I'm cruising the highway at "65" at "1800 rpm in overdrive at a certain indicated eco/mpg. When I come to a grade that tends to lug the engine below a certain rpm, it shifts down.....after perhaps lugging down a bit, slowing down, loosing headway and dropping eco/mpg. If I'm towing it for sure happens sooner. If I push the o/d off button, I get a burst of increased rpm/hp/road speed doesn't decrease as it was.
All you're doing is running in a different rpm range. One squeezes out a little better mpg at a cost of shifting/wandering auto shifting.
On trips with lots of up/down grades, I find it useful to switch o/d off.
My 08JK doesn't have a separate eco button.
By dropping down into a more productive portion of the power band, you will sense a stronger pull.
I use mine as a sort of auxillary gear, sometimes for better pull, sometimes for better eco. They are designed to run in either mode.
get in--buckle up--hang on
"soothing agricultural implement/personal servant/Walter Mitty multidimensional access utility device conveniently travels on pavement when necessary and often keeps me warm/cool/dry/soothed as needed."