I have a 2011 Ford Fiesta with an automatic transmission. When are you supposed to use this button on the side of the shifter?
Do you mean the switch described on page 247 of your owner's manual? If so, that would be the overdrive cancel/grade assist switch.
Pressing that switch deactivates 6th gear to eliminate some shifting in hilly terrain. As explained in the owner's manual, deactivating 6th gear extends lower-gear operation going uphill and provides additional engine braking when going downhill. A light in the instrument cluster is supposed to illuminate when the switch is on.
Ford recommends turning off the overdrive cancel feature on flat ground for best fuel economy and transmission performance.
Some automatic transmissions from other manufacturers also have overdrive lockout switches that deactivate the highest gear.
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