Just as a follow up and close to this thread, I delivered the Cherokee to our Son in Oklahoma......here are some highway mileage results using a 180F t-stat. 1st leg of the trip was 218 miles on 9.8 gals, 22.24 mpg. Ambient temp ranged from 42F to 54F, ScanGauge coolant temp floated 179F to 183F, speed was 70 to 75 mph, no cruise control as our Cherokee is an SE (stripped edition). 2nd leg of the trip was 239 miles using 10.9 gals, 21.92 mpg. Ambient temp ranged from 54F to 45F, coolant temp still 179F to 183F, speed was 70 to 80 mph (I44 in OK 75 mph legal).

Again, I don't think a 195F t-stat is mandatory for the 4.0L to achieve decent fuel economy. Will a 195F t-stat give better fuel economy, maybe, but I doubt it and I'm not going to put a 195F back in to find out. Is it necessary for coolant temp to reach 195F before the system will go into closed loop in order to achieve decent mpg's.......NOT.