I don't have the big tow pkg, and didn't want it because I wanted the better mileage of the taller gears, and didn't need the wide mirrors to see around my Civic on an open trailer. With my load, I pull in 5th and it almost never downshifts. If I hit hills, I put it in 4th for a while, no big deal. I have tested back to back on full tanks of gas on the same day running in 4th vs 5th and there wasn't any difference in mileage. It is louder in 4th, obviously. The trick to keeping any truck from downshifting from overdrive is to get you speed up enough past the natural 4-5 upshift point. Unless you're pulling something with a high wind load, the transmission hunting should improve with speed. Enclosed trailers are NOT the same dynamic, as the wind load is the dominant factor.
Thanks, that's good to hear, that was pretty much my thinking that I was hoping would happen. So approximately where is the "natural 4-5 shift point"? Also, just so I understand what you're saying, when you tested 4th vs 5th, were you going the same average speed?
Edit: Also interesting point about the mirrors, I figured I'd have to get some tow mirrors afterwards. How wide/long is your trailer?
I don't know off the top of my head where the shift points are, but if you were trying to run at 50-55 in 5th with a trailer, it's probably going to try to downshift a lot. Running at 70-80 there is no problem unless you hit a big hill.
When I tested 4th vs 5th I ran at the same speed. I think it was 70. I've actually done quite a bit of mileage testing with it, mostly because I'm a geek. Without a doubt, the most important factor is wind. If you can find a truck or bus that is going the speed you want to go, you will pick up mileage by following it. The improvement gets better the closer you follow, but even a very reasonable distances (ie, nowhere close to tailgating), there is still a benefit. I installed a scangage from work for one of my trips to Blytheville from Ohio, and in full on hypermiling mode, I was able to get almost 16mpg towing. This was at around 65, trying to follow trucks and being careful with hills.
My trailer is very small, just big enough to fit an 86 Civic. My garage is really tight, so the extra mirror width really would be a problem. I'd probably have to use the power folding feature every time I pulled in.