Well, there's a few things to consider...
I'm going to wager that your tow vehicle doesn't have 12" tires, probably more like 15"+. That means that if your vehicle is traveling at 55mph, and you have smaller tires on the trailer than on your vehicle, the wheels on your trailer are actually traveling at a higher RPM, therefor they are actually moving at an equivalent speed greater than your vehicle :shock:. That said, they're still moving at the effective speed of the vehicle, just at a higher RPM. You'd have to do the math to figure out how exactly how much faster :?:, but what I'm getting at here is that your trailer, with a smaller wheel diameter, is actually seeing more wear (that means tires, bearings, grease, all components of the trailer's system) than your vehicle is.
Now, theoretical mathematics aside, what's a safe speed? Well, that depends on a number of factors...load weight, traffic conditions, road conditions, weather conditions, etc. Most of the time when I'm towing my boat/trailer I'm going on a weekend camping/fishing trip 150-200+ miles from my house, into the mountains (6000-7000ft elevation gain), and with a loaded vehicle and loaded boat (vehicle full of passengers + some gear, boat full of the remaining gear and passengers who complain 8) ...just kidding).
For the most part, the lakes I frequent are off of well maintained (major) highways with speed limits of 65-75mph. With no boat/trailer/load I easily drive 75-80mph on these highways, loaded with a trailer I keep it to 60-65mph, more like 60mph. I'd welcome 55mph, but at that slow of a speed I seem to be impeding traffic rather than traveling at the slower end of it. Although it may be safer to tow at that speed, it becomes more dangerous being the really slow guy amongst the rat race speeders. You have to keep your head on your shoulders and driver smarter, not harder. :wink:
Obviously, you'll also need to adjust your speed (slower) for weather, road conditions, traffic, etc.
I'm kind of anal about towing, but people seldom realize how deadly driving an automobile is, and how MUCH MORE DEADLY towing is, rather, can be. That said, I check my load (boat tie straps, tarps, netting, etc.), safety chains/hitch, and tires/hubs whenever we stop for a nature break, food, etc. It's just good practice. I'd rather catch a problem early, then be subject to a roadside emergency...it's that ounce of prevention, pound of cure philosophy.
Anyway, I don't mean to be so long winded, I just wanted to take the opportunity to remind the readers that towing is a dangerous thing...too many people don't give it the respect it deserves...know your rig, know your trailer, know your load, don't be a "sheeple". 8)