Overdrive on a my 95 Ford thunderbird?

I have a 95 Ford thunderbird… The question is, what is the overdrive for… I know on trucks its used to pull trailers and go up steep inclines but I don't know why I have the option to use it or not on my Tbird. And what will happen if its on? Its the V6 3.8.

That's not what overdrive is for.

Overdrive is a higher than normal gear that you use on the open road once you get up to cruising speed. It improves economy and makes less noise, so it's a good thing. When you get to a hill you probably change OUT of overdrive to get more power. (same with a truck)

The term isn't used so much now, but the high gear in most modern cars in actually an overdrive (more than 1:1 ratio) With 6 or 7 speed gearboxes there may even be 2 overdriven gears.

As posted, a greater than 1:1 ratio would be 1 to 1.3.

so your engine can spin once, but your axel spin 1.3 times, thus being more fuel efficient.

You are totally wrong about overdrive. You NEVER use overdrive to pull trailers or go up steep inclines. Over drive is a high gear (4th) that has a gear ratio higher than your actual engine RPMs. It is to save fuel. Under normal driving use OD. Under extreme driving use 3rd.