Good chart for determining your RPM at any given tire size.
Where you would like to fall is in the "black" zone.
The "blue" zone is going to possibly affect your highway MPG and have you turning a few more RPMs than you may desire.
The "yellow" zone may not give you enough highway RPMs but most likely give you better low-end grunt.
You will want to factor in your engine's torque and horsepower band to get better use of this chart.
Jeep 4.2/4.0 L I-6:
Horsepower (net) 185 @ 4750 +/-
Torque (net) 220 @ 4000 +/-
Jeep 2.5 L I-4:
Horsepower (net) 123 @ 5250 +/-
Torque (net) 139 @ 3250 +/-