Nothing wrong with going the speed limit as long as one knows their combo well enough to be able to respond/react quickly enough in case they have to make a sudden stop or an emergency maneuver. I get my best mpg. towing at 55-62 mph, but I've run at 65 and 70 mph in those speed zones on occasion. Secondary roads are 25-55mph anyway, so no faster in those speed zones..65-70+ mph really drops mpg, so in my case, max I would tow at would probably be no higher than 70mph, more for safety factor reasons and better fuel economy than 70+ mph.