Since I can't afford to run 93 octane (87,89,and93 are the only available here in PA) I started running 89 octane, but left my Superchips 91 octane tune in. I have noticed no problems at all. I was afraid I would get detonation, but have none. I also have a 180 thermostat, maybe that is helping a little by adding a touch more fuel and keeping the temps down a touch in the combustion chambers. I get between 13 and 14 mpg out of my 2500 qc 4X4 with the hemi. Can get 14 to 15 if I run the 87 tune and run 89 octane, but I lose alot of power from the 91 tune. Anyone else have any experience with running 89 octane on 91 tune????

 

I used to do it on my V10. Never noticed a problem.

Just listen for any early detonation or pinging. The only place you may notice it is on a good hill under a load.