Math
I found a formula on some 4x4 site(can't remember the name) but it will give you your RPM at a given speed depending on what gears and tire size you run.
here it it!!
speed x gear ratio x 336 / tire size
I'm not sure were the 336 comes from it is just part of the formula, and the gear ratio would be your diff gears in a 1:1 ratio or 4th gear in a manual, 3rd gear in an auto. But you could just multiply your diff gears by your over drive gear ratio.
Ex. (60mph)
60 x 3.73 x 336 = 75,196.8
75,196.8 / 27 = 2,785 RPM
This would be my RPMs in 3rd gear with stock tires NOT over drive
Ex. (60mph in OD) Diff gears x OD gear ratio in A4LD auto trans
3.73 x 0.75 = 2.7975
60 x 2.7975 x 336 = 56,397.6
56,397.6 / 27 = 2,088.8 RPM
I think its pretty damn cool