My redneck engineering part of my brain says: the manufacturer is figuring the tow capacity in the WORST CASE scenario for extended periods (hundreds or thousands of miles). That's any combination of freeway speeds, twisty roads, inclines / declines, inclimate weather etc all. So, I think it'll be fine if you keep in mind you're 140% over rated capacity (short distance, drive slow, on levelish ground, surface streets only, break early, driver uber-defensively etc).