The previous owner of my F250 diesel towed a 14K loaded fifth-wheel to Florida and back (to Nova Scotia) every winter. Way over the rated load capacity on pin weight, but he loved it, thought it did great. For me, around 9K - 10K is where there is a dividing line between gasser and diesel. Above 9K my Dodge 2500 with Hemi and 3.73 gears kind of gives up - can barely maintain highway speed in 4th gear on easy hills, slow in third gear on harder hills, and 7.5 mpg. The Ford is in fifth gear 90% of the time, down-shifts to 4th once in a while on a hill and gets 11 mpg with the same load on the same road.
