I pull a 5000 lb horse trailer all over the eastern US loaded with 2 1000 lb horses and only take overdrive off when in the city. Never had a problem.[8D]

 

I towed a 2000 lb pop-up camper from central PA to Washington D.C. the other week. Fair amount of hills, so most of the time I found it was better to run with the OD off. Also I was following my father-in-law so I wanted to keep the gearing down and the power up, because I never knew which lane he was going to jump in and I wanted to stay on his bumper. Anyways, the trip was around 400 miles round trip, mostly highway. I dropped 1-2 mpg (13.3 avg trip mpg), which I figured was pretty good for towing. So if you find the tranny is hunting gears or you can't keep the speed, leave the OD off and don't worry about losing much in mileage.