sooo? i have it in my car and i alway drive in od and i didnt know what it was till today...what is it for?

 

Its basically a "4th" gear in the 4L60E, that reduces engine RPM and increases gas mileage at high road speeds. The gear ratios in the 4L60E are shown here:

http://www.ws6.com/faq/4_3.htm#ch4_3a

Direct drive is 1.00:1 (3rd gear in the 4L60E) where the driveshaft RPM is equal to the engine RPM (locked converter only). Overdrive is any gear that has a lower ratio than 1.00:1. In the case of the 4L60E, 4th gear (O/D) is 0.70:1. That means that the engine speed has slowed down 30% for the same driveshaft speed. The car travels further for each turn of the engine because the driveshaft speed is OVER the engine speed.

When you put the selector lever in OD, you are allowing the transmission to utilize all 4 gears, based on the shift values programmed into the PCM. Watch your tach as the tranny shifts.... it should shift 3 times. As it shifts from 3rd to 4th (OD) the RPM will drop 30%.

 

ok cool thanks for th einfo injuneer....so when im driving around town and whatnot should i drive in od or d?

 

Just leave it in OD, the tranny is electronically controlled and will (almost) always make the best decision for the proper gear to be in.