With a ford 150- I start the car and overdrive is on and my freinds tell to turn it off. So should I leave it on or off- and what is it even for?

 

Default is on which means it will shift into overdrive 5th gear. You should turn it off when you are towing.

 

it's you're final transmission gear, gear ratios are based from direct drive 1:1 under drive gears are gears that are lower than 1:1 so if the engine crankshaft spins 2 times for every 1 rotation of the driveshaft your gear ratio would would be 2:1, overdrive would be on the other side of the spectrum where the crankshaft speed is lower than the driveshaft speed. turning off overdrive is generally only usefull if you are towing or driving up a steep hill