What does overdrive do?
 

What does the overdrive button on my jeep jk unlimited do? Will i get better mpg if it is off or
On? What does it effect?

I have 33" with 3.21 gears and an exhaust

It's the 4th gear and last gear in your transmission. The main purpose of the button is to lock out 4th should you want to keep the engine rpms higher like if you were towing or going up a long hill and the jeep kept bogging down and it was downshifting frequently. Since you live in flat FLA that's probably not an issue. So unless you find the jeep "hunting" alot between 3rd and 4th I'd just leave to OD on for the best mpgs. If your looking for better mpg's slow down, the JK is a brick and I know it's hard not to do 75-80 mph down there but that's the biggest killer of mpgs.

I tow (boat or camper) and it has to be turned off (OD Off), limiting the number of gears to 1st - 3rd. Otherwise as mentioned above it will go between 4th gear and 3rd frequently (depending on the terrain).

I have to say, they put that button in an inconvenient location. At least it is in my 2010, I'm not sure if its been relocated or not.

You have 4 gears in your transmission. Overdrive gear is the 4th. When overdrive is off, the transmission won't shift into 4th. This means you will run a lower gear when normally you'd be in 4th. I would bet that in your case you will see improvement in mpg. Best thing is to experiment as there are different variables involved.

So should i keep overdrive on or turn it off in your opinion? Will it harm anything if its on or off?

Leave it on till your transmission start shifting a lot or in other words looking for gear to stay in about 50 miles an hour.

Experiment with it. See where you get better mpg. If it shifts a lot, then definitely leave it off.

It's a Jeep. Push the button and it lets you drive-over things.:rotflmao2:

Overdrive is any gear higher (numerically lower) than 1.0:1.

JK's in the 2007-11 range have a 4 speed auto with the following gears:
1st - 2.84
2nd - 1.57
3rd - 1.0
4th - 0.69

Usually any time you can gear higher than 1.0:1 and stay in the appropriate power range of the engine you are going to get more efficiency out of the engine (ie. better gas mileage).

Most 4 and 5 speed auto tranny's are single overdrive transmissions meaning they have only one overdrive gear. 6 speed and higher tranny's are generally double or tripple overdrive tranny's.

As most have already said turning off overdrive simply electronically prevents your 4 speed transmission from being able to shift into 4th gear (overdrive). You would generally do this to prevent the transmission from "hunting" (shifting up and down) for a gear when pully a load or going up a significant grade. When overdrive is off 1st-3rd work as they always do.