What kind of rpms are you guys running down the highway. I have 285/75 and I crank about 2200-2300 @65mph.

Your corrected speed at 65 mph on the gauge is 70.
Here's a tire calculator:
http://www.miata.net/garage/tirecalc.html
For comparison, at a corrected 62 mph my tach reads 1500 (58 mph on the speedo)
This of course is all expecting you are running a Duramax/Allison with 3.73:1 gear.

That makes sense