89. especially when its hot out. octane helps it run cooler.
but 89 mostly
I think i saw stock comp on my 2001 4.0 was 8.9:1 compression. thats way low enough to run 87. i go back and forth depending on my mood. But I have probably never had a strait up 87 mix since theres always a bit of 89 or 91 or even 93 in the tank. I dont go below 1/4th tank since i live in BFE.
i know my 2001 f-150 v6 really hates 87 but likes 89. so thats my reasoning...peace of mind even though science says im fine with 87
2001 TJ 4.0L Automatic, D35/30, both hard and soft top, full doors,JKS quicker Disconnects, 31" discoverer ATR, American racing 767 offset 15" wheel. ProComp Genuine steel bumpers front and back. glass pack muffler, Rugged Ridge extended fender flares, 130w KC daylighters, motorcycle light for reverse lights. 86,000k miles (11-7-2012)