Hmmm I have an 07 4.7 liter and just hit 150,000 miles today. I have been running the 91 performance tune for well over 3 years with either 91 or 93 octane gas and haven't had any issues! What plugs do you use. I've found out that the OEM champions work best for my jeep. I'm getting almost 13 mpg city and I'm running 305/70R17's. This weekend on a 6 hour drive to play Army I got up to 16.8 mpg with cruise set to 65 mph.