I have the stockers and at 75mph I'm hitting 2100 or 2200 rpm. What tire size would drop below 2k but not to much? 285/75? or would a 275 do it?

 

315/70R/17s will put you around 1900rpms at 75mph with 48re and 3.73 gears.

285/70R/17s will put you around 2000rpm at 75mph with 48re and 3.73 gears.

 

265/70-19.5 puts me at 75 at 1900 RPM. Bridgestone R250 tires roll nice on the Highway..

 

If you increase tire size to much with stock rear end you can lose some power in taking off especially with a load. I'm running an '03 dually long bed, 305 and 48RE with a 3:73 rear end and turn around 2000 to 2100 at 75. Truck really hooks up around 78 to 82 when everything smooths out. That's about where I run it most of the time when on the highway.