I'm going out to look at a new F350 (sorry, a Ford guy, it's a genetic thing) diesel w/ automatic and an EZ Dumper insert today. Will be pulling a 6 X 12 tandem axle trailer w/ a 7000 GVW. My questions are - will a dually increase my weight carrying capacity that much? Cost for a dually isn't that much more so is it worth it? What about gas mileage difference for a dually?