i'll turn on OD at about 55. at that speed with OD off the motors turning about 2000 rpm and with OD on its about 1500. anything less than 1500 rpm and the truck its a pig.

 

It is a matter of preference for sure. In my view, I just think it isn't necessary. All of that excess motor noise and engine braking.

 

Turning the O/D off just locks O/D out so the tranny can't shift into it. As soon as i get in I turn mine off and drive it even running 70mph O/D off I get better mileage.

Truck: 97 1500 4x4
Engine: 5.2
Tranny: Autotragic
Gears: 3.50:1
Tires: 33's
O/D on: 9mpg
O/D off: 14mpg

On any smallblock your best econimy will be at 2475rpm anything under 2000 is just bogging the engine. Its a smallblock the old chargers turned 3500-4000rpm all day with no issues.

 

Like MonteC said, it does not change the shift points, it can't. The computer doesn't control the shift points, just the converter lock-up and O/D shift.

I turn mine off as I'm accelerating onto the interstate. It usually kicks in at around 55-ish, depending on how much throttle I'm giving it. I don't mind it being a pig in that situation, as long as I ain't trying to merge into a small gap in traffic or something.

 

I didn't mean to say it goes into OD at 40-45, I worded it wrong and just meant driving around with it on at those speeds. And trust me , with 3:55's, theres no engine breaking or engine noise. I believe that having it on at lower speeds just creates more heat and wear and tear on the tranny. Between turning OD off and having a factory cooler is what I attribute it to lasting as long as it has. 149K knock on wood, LOL

And as far as the MPG go. I only have 265's on it . And I know I'm not the only one that gets crappy MPG, I've seen plenty people here and other sites saying the exact thing.

Actually , after the TC locks up it bogs badly enough . I'd like to keep it from locking up at all, lol . Probably a way of doing it ,but then I'll prolly get a light.

 

Cooler flow actually increases when the transmission goes into o/d and lockup. When lockup comes in it locks up the converter and greatly reduces heat. 90 percent of the heat generated in the transmission is in the torque converter. All this means that when your truck goes into O/D the transmission is actually getting cooler.

 

Does your truck engine brake in third gear? It shouldnt.

 

Actually, it does when the transmission is in O/D and when speeds are above approximately 30MPH.

The owner's manual states that the transmission has an electronically controlled fourth speed. The FSM states:

The torque converter clutch is hydraulically applied and is released when fluid is vented from the hydraulic circuit by the torque converter control (TCC) solenoid on the valve body. The torque converter clutch is controlled by the Powertrain Control Module (PCM). The torque converter clutch engages in fourth gear, and in third gear under various conditions, such as when the O/D switch is OFF, when the vehicle is cruising on a level surface after the vehicle has warmed up. The torque converter clutch will disengage momentarily when an increase in engine load is sensed by the PCM, such as when the vehicle begins to go uphill or the throttle pressure is increased. 2001 FSM 21-480

and

Fourth gear overdrive range is electronically controlled and hydraulically activated. 2001 FSM 21-486

Perhaps I am phrasing it incorrectly, but it does change where/when the engine shifts while in O/D. It does not affect 1st through 3rd or reverse shifting, except that it will engage the O/D at speeds above approximately 30MPH. It will also change how often it shifts based on engine load or throttle position by the PCM disengaging the torque convertor clutch or O/D. This will affect how and where the engine shifts.

 

Any time the torque converter is locked up it should run cooler due to eliminating the slippage. At least I'm under that impression but I could be wrong as I have been many times in the last 72 years & 11 months.

 

If it would stay locked up in O/D, it would run cooler. The problem comes from the many situations where the PCM will disengage O/D and downshift. More frequent shifting increases heating.

From the owner's manual:

I have an addendum to my owner's manual which also indicates that O/D will not work when the ambient temperature is less than -5ºF. It also indicates that it will turn off O/D if the transmission temp exceeds 260ºF.