I have a 2003 Ram 2500 Hemi, and while it's a manual 5 speed it is never below 50 mph in third gear on any highway (interstate type) grade up to 6%, towing 8K lbs. The stock tires (on my truck at least, which is 2wd) are 245-70/17, so your tires are probably the problem. You have effectively geared it down significantly - almost 10%.
My Dodge can tow that load without a problem, but it sure can't pass a gas station. I get around 7.5 - 8.5 mpg, hence I switched to a diesel for towing, the Dodge is now my work truck.
Brian
