You guys are a lot braver than I am. I've seen way too many of those flimsy cables break under less stress than the weight of an ATV. I won't even load my ATV on my tailgate let alone go down the road with it. I remove it, place the ramps on the bumper and load it, then put the tailgate back on.
We loaded a large gate valve on the tailgate of the work truck one time (2010 chevy half ton) and it folded the tailgate. Valve was only about 600 lbs.
Most cables on full size trucks have a safe working load of 800 pounds. So technically they are good for a combined 1600 lbs. (compact trucks have smaller cables) This is assuming the cables aren't frayed, pinched, or rusted. Kind of hard to do an inspection when they are covered in a protective sheath.
Now say you have a 650 pound machine, 200 pound rider for a total of 850 pounds. When the entire weight of the machine and rider is on the ramps and the tires are about to transition to the tailgate, how much weight is being placed on the tailgate? I imagine it is easily 2/3 or more of the total weight, which technically is still well within the means of the cables if they are in good condition. Now that the machine is loaded, say the rear tires are sitting in the center of the tailgate. So about half the weight or 325 pounds is now on the tailgate (more if the machine sets closer to the edge of the tailgate due to leverage on the cables).
1300 pounds seems like quite the safety factor and it is, except for the shock load you are placing on the cables going down the road at 70+mph. Under shock load, Lifting devices are only rated for 10% of their static load rating, meaning the cables on your tailgate that have a SWL (safe working load) of 1600 pounds are only good for 160 pounds under a shock load. So as you can see, there isn't as much of a safety margin as most think. I know a lot of people do it and get away with it, but there will be failures from time to time. Might not be a bad thing if you need a new tailgate anyway... :thumbsmilie:
Just make sure she's strapped down real good!