I picked up a 1999 3500 dually with the 5sp manual.

Running down the highway at 65 to 70mph i am turning 23-2500 RPMs. going 75 is basically not an option, because i dont like running the cummins at 2800 rpm's

1st gear is completely useless unloaded, and i really dont need to use it even with our 30ft 5th wheel behind it. 2nd gear is Plenty low enough for starts.
I thought that it had 4.10's in it. But maybe its has 4.55's? it is used.

I do a lot of highway driving and would like the motor to be turning alot less rpms at speeds of 65 plus. and be turning around 4-500 rpms lower at 65. as that is the max speed i tow at.

What kind of revs are you guys running at highway speeds?

What kind of drop would i see by dropping to 3.92's?

 

Do you know which differentials your truck has? The newer ones use the AAM 11.5" rear, but I think yours probably has a Dana 70 or 80. The website for Randy's Ring and Pinion can help you identify your diffs. For the Dana axles there is no 3.92 available. You could buy 3.73 or 4.11 gears.

You could check to see what your current gearing is by jacking up a rear wheel and see how many revs you get out of the driveshaft by turning the wheel one revolution. Put a piece of tape on the shaft.

 

By the way, mine turns about 2150 RPM at 70 MPG in 6th gear.

 

It has the Dana 80 in it. Its in the shop for new calipers and the mechanic said it has the 4.10's in it.

Ill check out that website. thanks for the help. Now to find out weather or not 3.73's will be too tall.

 

Nah 3.73 won't be too tall. You might even think about 3.55, they are available to for your year truck.

 

Yeah, I would think about 3.55s too.