Quote:
Originally Posted by ballisticmike
hmm, interesting.. i didnt know about that stuff. Is OD the same as fifth gear? Does comgear show OD? I like to hit the OD button and turn it off ..
1. when on windy/hilly roads
2. going down long hills to control speed
are these mis-uses of the OD button? I know that button is mainly intended for trailer towing
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Overdrive just means the tires rotate faster than the engine.
Here are the ratios for the 4R75E transmission.
Gear-engine speed:tire rotation
1-
2.84:1
2-
1.55:1
3-
1.00:1
4-0
0.7:1
So by looking at this table the 4th gear is called overdrive because the tires rotate 1.43 times for every time the engine rotates once.
Hitting the overdrive button will just keep your truck from going into 4th gear. If you are towing something and you hear/see/feel the truck shifting between 3rd and 4th constantly, you should hit that button. It is there to keep the transmission from shifting more than it needs to. If you are going down a long hill in just the truck, yeah, you can use it to slow down without touching the brakes. By doing that you force the truck into 3rd gear and start an engine brake. I've heard that engine braking with these lighter trucks isn't good if you have a really heavy load, but just the truck is fine most of the time.
You will also notice that when you hit the OD button the COM GEAR will change to 3 with the lock. This means that you downshifted to third and the torque converter locked.
*Oh and BTW, those numbers are before your axle. I was using a 1 to 1 axle ratio to make the differences easier to see. Otherwise I'd add in the typical 3.31:1, 3.55:1, 3.73:1 or 4.10:1 axle ratios.