got a quick question guys
 

I have 05 dually 4x4 and when I hit 60 I am shooting about 2000 rpm. My axle ratio says 4.10 but anybody know how I can reduce the rpms?

Bigger tires, gear vendor overdrive or swap gears

Yup, pretty much it. I'm running 35x12.5x17's with 3.73 gears, 2k rpm for me isn't til about 75mph. The larger tires actually helped my mileage, since I do a lot of highway driving.

+1 with Mdud....jump up the tire size to tame the RPMs. Low end may be affected somewhat but the drop in HWY rpms will make you happy. IIRC DRWs are a 17" wheel so look at Nitto Terras at 285/70/17.

The dually rims are REALLY narrow though aren't they? 285's would probably be ok, but anything larger might need different spacers. I think... haha

I'm just curious how do bigger tires make a diff? If someone can explain please?

A bigger tire will travel farther in one rotation than a smaller tire. I can't remember the math off top my head, but just throwing out numbers, a stock size tire will rotate 500 times in a mile. A tire two inches taller will rotate 475 times a mile. So by adding a bigger tire, u have just lowered your overall drive ratio, which lowers rpm.

It's just simple gearing calculations. Obviously going too big will offset the other way since it's a TON of rolling weight you'd be adding. Luckily we all drive diesel trucks that have enough power to turn these tires over with ease. Like GreenGoblin said, a taller tire will rotate less times in a fixed distance, thus lowering your cruising RPM's at a given speed.

Don't forget to get the speedo recalibrated so you can accurately measure your mileage. If the mileage is off, your MPG calculation will be off.