Is it true that for best highway MPG you want a certain kind of gearing, and for the best offroad/towing you want the exact opposite kind of gearing? Is there a happy medium?

 

Yea that's pretty much true. 3.21 gears get better mileage than 4.10's in general because the RPM is lower for the same speeds (when you compare the two). 4.10's however are MUCH better for offroading. Although you can get away with 3.21's as long as you don't go too far with lifts and tires and added weight.

The happy medium is 3.73. These are all gearsets offered for JKs.