Its not a Turbo option, rather that button is for "Over Drive" O/D Mode.
Overdrive is the operation of an automobile cruising at sustained speed with reduced engine revolutions per minute (RPM), leading to better fuel consumption, lower noise, and lower wear.
Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached.
When it is off, the automatic transmission shifting is limited to the lower gears.
Overdrive should usually be selected when the average speed is above 70 km/h.
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