The truth is there's no speed below which everything is peachy keen and above which disaster is certain to strike. But there's definitely a zone that's considered safer and a threshold where things start to get dicey.
The maximum controllable speed (or maximum open road speed) is the speed at which an outside factor, maybe a gust of wind or the need to make a sudden maneuver, will cause a vehicle to go out of its operator's control. There's not a way to set a precise mile-per-hour limit though, because the maximum controllable speed depends on a couple of big variables -- namely the vehicle, the driver and the driving conditions.
For example, if you're just setting out on a tow, take it easy and test the waters. A bad packing job might mean the maximum controllable speed your trailer can handle is lower. If it's a gorgeous day and you're an old hand at towing, you can probably muster the trip a little faster. Bumpy old roads mean you have to slow down, so does the company of big trucks and cross-currents. Every time you see your trailer start to sway, ease off the gas until it straightens out.
It's also important to keep in mind there may be government regulations in place that limit how fast you can drive with a trailer. Before you head out, you'll probably want to check out the rules and regulations enforced by local transportation agencies to avoid a ticket.
For example, if the HowStuffWorks crew decided to spend an afternoon boating at a nearby lake, our only guideline for how fast we could drive to get there would be the posted speed limit and the trailer-towing skills of whoever was doing the driving. If we were boating in, say, New Zealand, we would be legally limited to between about 50 and 55 mph (80 to 90 kph) even if the highway signs said something higher. U-Haul recommends a maximum speed of 45 mph (about 75 kph) for users of their rental towing equipment [source: U-Haul].
So just remember, no matter how badly you want to get where you're going, anyone with experience in towing will tell you that's a recipe for disaster. Towing calls for care and concentration, not quickness.
For more information on planes, trains and automobiles, see the links on the next page.