Originally Posted by Twoxentrix
I leave mine on all the time, and only diseguage when going up a long steep grade (and some instances on down-grades to keep from having to ride the brakes a lot)
Essentially it takes the Trans out of overdrive...
This. Twoxentrix is saying he leaves OD on all the time.
Just to be clear I would call this Tow/Haul mode mode off. Ie, the light on the shift stalk is off unless you are towing or climbing, etc. This is what the owners manual recommends.
The 4r100 transmission has OD 4th and 1:1 3rd gears. Locking out 4th all the time would be like going back to 1970s 3-speed transmission tech. There is no way keeping OD locked out would improve fuel economy. Running higher engine speed also puts more cycles (wear) on valves, rings, cylinder walls, bearings and so on. You would have sportier performance with OD locked out all the time, which is perhaps why they told you to leave it in tow mode. Hey...they’re not the ones paying for the gas.
Using tow/haul mode does make sense when there is high engine load or the trans is frequently upshifting/downshifting. Higher engine and transmission speeds results in lower torque through the gears and also higher cooling system flows, which will help keep the engine/trans in the right temperature range.
I leave tow mode off (OD on) in my van when I tow the boat unless I’m headed up/down a hill. There is no reason I see to spin the engine faster on the flats, otherwise it’s just a waste of fuel.