I would say that's not true.
first, your transmission won't shift to OD until about 45, so below this, it won't make a single difference. (might be a bit lower if you have 4.10's)
above that, your transmission is set up to downshift before it gets too low in the RPM range, so it should not make a difference there either.
if you're towing something more than, oh, say 5Klbs or so, I would shut it off because i've heard the OD clutches aren't really made for towing. don't know how true this is though...
I would also say false, I have had one for over 20 years I'm the original owner and my truck was the first month production E4OD and I have never drove it with the od off. The only thing ever done to my trans has been fluid changes about every 30,000 miles. It has a 4.10 axle also
I have heard the e40d and 4r100 in the superdutys are built to tow in overdrive. The only reason you should take it out, is if ur towing at a speed that tranny is shifting in and out of overdrive a lot. That will excessive heat and wear. Thats what i have heard and read before.
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