I would think that RPM would dictate. Depending what size tires you run, 11s,14s,16s, etc, you can build more road speed. But the limiting factor is still rpm you run to high you hurt the motor.
You should also look at any mods that have been done to the engine, and what your carrying. Do you really wanna be running 56 mph with 30,000 pounds of stuff on a trailer. Some of the guys on here have the exprience to do it some dont. Do what you know you can do and have fun, get pics, Captain