I tow four down, and my car's manual tells me not to exceed 65 mph when being towed. With your Kar-Kaddy, I'm wondering whether the 55 mph statement is based on the speed rating of the dolly's tires? It appears that the Kar-Kaddy's tire is a ST205-75R-14 Load Range C trailer tire. I don't know that much about tires, but aren't most load range C trailer tires supposedly limited to 55 mph? That's my recollection from my old travel trailer days. Perhaps someone with more tire knowledge can weigh in on this.