I have a 92 3.9L dak.

Can some one explain what OD does?
Also when should I use it?
When should I NOT use it?

Should I use it when I tow my 14' boat?
Or when I am going up hill a lot thru the mountains?

 

Your truck has a 4 speed automatic. The first three gears are automatic--you can downshift with the shift lever by taking it from "D" to "L", but it will do the shifting for you otherwise. Overdrive the fourth gear, and you use it for normal driving on relatively flat ground and with only minor loads. If you get into the mountains, are pulling a boat, or have a load of bricks in the bed, you should disengage the overdrive--and use the lower gear range. The engine will turn a few more RPMs, oil pressure will be higher, and the engine won't ping or build heat as much.

And going downhill being out of OD will give you slightly better engine braking and tend to save the brakes from wear and heat (hot brakes are really, really bad.)

If you have a really heavy boat, you should get a brake controller and have your trailer brakes connected, btw.

 

Most of my driving is highway. So I should use OD for highway driving? How about in city?