Overdrive button on '00 Cougar
 

I just bought a 2000 Cougar coupe with an automatic transmission and wanted to know what the button on the left side of the gearshift does. I'm pretty sure it's for "overdrive," and am looking to see if it's best used for interstate highway driving or from going 0mph to 60mph quickly. For all I know, Mercury could've put this button on the gearshift just because it looks cool.

(I need to find an owner's manual for this Cougar.) Thanks in advance for everyone's help.