I test drove a 06 dually 6speed and I hit 4 grand on the tach really easy going through the gears..I wasnt getting on it that much..Is this normal or is the clutch going bad??..I got a 6speed right now with 38 inch tires on it and I can barely hit 3 grand.

 

Was it pulling hard when going to 4K or did it feel like a slipping clutch? Also it sounds like there is a programmer on the truck. A stock '06 5.9 shouldn't get much over about 3500 rpm. I know red line is 3200 but mine in stock mode will over run to about 3500 if you push it hard enough.

 

it didn't feel like the clutch was slipping..It has a air intake and gauges already on it

 

As a guess I'd say the tach or CPS is tango uniform. These things don't just go to 4k that easily.