My 5 year average towing a Jeep Cherokee weighing 4,000 pounds is 8.5 mpg with an '03 V-10 Winnie. I rarely drive faster than 60 and I'm happy to go up mountain passes at 35 mph. If you can stand the racket, you can go up the passes faster than me but you'll get even lower mpg. The majority of my driving has been in the West where we have the real mountains. If you're really getting 6 mpg, someone probably has a lead foot. The transfer case in my previous '89 Cherokee was very reluctant to be shifted into and out of neutral but my "new" '98 Cherokee shifts in and out happily. Try shifting the transfer case when the tranny is in neutral and without the brakes on. If that doesn't work, try the same with the engine idling.