RVUSA
Orlando, FL, USA
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Joined: 01/04/2005
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The tranny fluid temps rise rapidly when you leave OD off in those years when you are towing. They will keep rising when you climb hills. The immediate risk to higher temps is the front seal failing and dumping fluid out.
Get a scangauge and use it, set one of the readouts for TFT and watch it. Get as big of a cooler for the tranny as you can find and a fan for it, and even a water mister if you plan on doing large grades.
You can also look into replacing the transmission cooler bypass tube with a kit. It is also one of the culprits in the fluid temps rising when not in overdrive. It's pressure driven and when pressure rises it allows fluid to dump back into the tranny instead of going to the coolers first. And of course when you run it with OD off the pump builds a high enough pressure to cause it to leak fluid back into the tranny. A lot.
If it hunts for a gear a lot when in OD just use this mod to fix it. Obviously dont forget to turn it off when you are stopping. A couple forgets and you'll remember easy enough though. 
mod link
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skipnchar
Topeka or somewhere else
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Joined: 12/17/2003
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Older torqueflight transmissions did not have any way to lock out OD. Not sure about newer ones but kind of hard to imagine every locking it out since if it can't use it, it will just not shift into OD. The nice thing about an automatic transmission is it's automatic. Put in in Drive and step on the gas. Use tow/haul if you WANT to though I seldom did with my diesel as the shift points were about right without it. Going down hill it can be useful for controlling speed but with newer transmissions that allow you to control ALL gears manually, that's a better option.
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2004 F-150 HD (Traded after 80,000 towing miles)
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hotpepperkid
Chino Hills CA
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AZ T&T wrote:I had a 2000 F350 (Fantastic truck!) and had to have the tranny replaced at 35,000 miles. I was told that towing with the overdrive on was the main reason. I was over a 100,000 miles on the 2nd tranny when I sold the truck and never had any problems with it.
I would definitely tow with it off.
I also have a 2000 F-350 its chipped and I tow with the OD engaged and I have 160,000 miles on the truck. The chip was installed when the truck had 30,000 on her
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WellShooter2
West Texas
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With the trailer you describe you would never see the truck go into OD unless you were headed downhill and with the wind. Anybody that has ever pulled a big trailer will tell you that that NEVER happens.
So your question about MPG is moot. Tow with it off.
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Mile High
Denver, CO
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Those were not known to be the best transmissions - On my 2000 the transmission went out at 55K miles. Use common sense but if you know you are loading it down I would turn the OD off.
The 7.3 was a great engine, backed by a not so good transmission. My current Torqueshift is a fantastic transmission, motivated by a not so good engine. go figure.
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sirdrakejr
Las Vegas, Nevada
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Moved from 5th wheels to tow vehicles.
Frank
2011 Palomino Maverick 1000SLLB on a 2004 Dodge Quadcab CTD Ram3500 SRW long bed equipped with Timbren springs, Stable Load bump stops, Rickson 19.5" wheels/"G" range tires and a Helwig "Big Wig" rear anti sway bar.
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Cool Canuck
Winnipeg, MB, Canada
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A quick read of the Owners Manual will tell you to leave the OD "On" unless it is hunting between gears. Towing with the OD On had little if anything to do with the 4R100 transmission failures. The Torque Convertors would not standup to the increased torque of a "Chipped" engine. Having said that, chipping the 7.3 makes for a very nice tow vehicle. Also FYI some 2000's had torque convertors that sounded like a can full of marbles.
I had my '01 for 7 yrs and it never left me on the side of the road. Not even a flat tire.
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dannytas
Pioneer, CA
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I have a 2003 Dodge dually with the same switch. I leave my on. As said previously, it won't start out in O/D. When I'm climbing a grade or going down hill I turn it off, because it starts "hunting" for gears. Hope it help, some 
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okiejoe
Oklahoma City
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Thats why I have a six speed manual, no overheating problems either, but I do know people don't like to shift so stay out of overdrive.
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davelinde
Lake Nona, Florida
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RVUSA wrote:The tranny fluid temps rise rapidly when you leave OD off
I found the opposite to be true.
When I was towing 10,000# I let the tranny shift into OD. It did not hunt and temps were OK. When I jumped to 14,000# the tranny would hold OD on the flats, but the transmission temp would creep up. When I locked out OD and force the downshift the trans temp dropped - so I would normally lock out OD towing heavy.
Dave Lindemulder
Tammy, Mark & Kirsten
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