I don't know of anyone with one on a cummins but I know some fords with them. I have looked into it and if I was running a rig that towed for a living I would get one. The 6 speed needs a gear between 5th and 6th bad. The double over would be nice when empty too. I saw a 2wd dually 4 door on ebay for sale with one and the guy claimed he got 28mpg empty.
I've got one it maintains good steady mpg's empty, a shade under 19, up to 84 mph @ 2k rpm's. I average 19mpg in every day driving always with some giddy up on the go pedal. One of the drawbacks with it on these old style g56 trans 5th over is practically the same final drive ratio as 6th under so there is no gear between 5th and 6th. I don't tow - well not anything of significant weight worth mentioning anyways - so I'm no help there but I can see where it'd be nice on mountain grades. So for me it's basically an overpriced toy I had to have back when I first got the truck. That's not to say it isn't nice I do like it but they are overpriced imo. As a matter of routine I run about 80, 85 sometimes even 90mph on the interstate and it always bothered me revving the engine so high to do that. Not to mention the crap fuel mileage it was netting. I know these engines will run all day long at 3000 rpms but not my engine! so long term maybe it'll thank me for it.. I love my 5.9 and I want this truck to last forever that was the whole reason I got one..
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