Steve: physically, you *can* do that...but the trucks aren't really designed for it...I looked into it also, but the cons seemed to outweigh the pros. Everyone that had done it said the ride was horrible, they made the truck wander, but more they couldn't stop as well because of all the additional rotating mass. If you're worried about you legal rated GVW, I've read where a few guys have had their trucks re-tagged for more GVW...like us D/A guys, you Cummins guys have the AAM axle, which is actually rated at around 11k pounds...but due to the cheap tires that come on our trucks from the factory, they're stuck to like 8500 lbs or so...so, jump up to an F or G rated tire, print the certification info off of the AAM website, and you can legally up your GVW, and your DOT can retag it as such...or so they say. Adding airbags is supposed to help, also...
And don't feel bad about the mileage...our D/A dually doesn't see 20mpg either...
