Pretty easy to disable the mechanical speedo. I looked at a 2006 3500 dually that claimed was 11K miles. Yeah, 11K miles was more like 211K miles. Had a ton of marks on the bed (under shiny new plastic bed liner) even had a 4" hole from the gooseneck hitch on it. Tires were near new, but they were not the factory size... Who replaces all 6 tires on a dually in 11K miles with one size bigger off-brand tires??? :-)
Had to pass on it, even though I wanted one of them without all the emission crap I have on my 2010. As soon as my warranty is up it is all going bye-bye.