Core XK's Garage

[06JeepXKHEMI]

I ran my HEMI always with 91 or 93 with the tune at 91 Performance tune. I was avg 17-18mpg highway even with the MT/Rs with I stayed under 72mph and 12-13 city with a heavy foot. My intake I cleaned constantly cause Im ocd about it. You really dont have to but with the wheeling, winter and constant city driving I like to keep it clean. Keep in mind I work from home and have down time where the filter can dry enough before putting the oil on.

[lekmedm]

I ran my HEMI always with 91 or 93 with the tune at 91 Performance tune. I was avg 17-18mpg highway even with the MT/Rs with I stayed under 72mph and 12-13 city with a heavy foot.

I get those numbers running 87 with my 4"SL and 285s and no engine tune.

[06JeepXKHEMI]

My HEMI ran like **** with anything less than 91 in it, even with the right tune and my mpgs were terrible. I also drove aggressive cause who doesnt like the sound of a HEMI at WOT with true duals!

[07JeepXK]

I get those numbers running 87 with my 4"SL and 285s and no engine tune.

Haha I get those numbers with my 305/70 MT/R's

[fco27]

Seriously the best I could get is 15.5mpg on the highway but I drive 75 to 80 mph.

[lekmedm]

I've done very well running 87 octane since new.

Sent from wherever my Jeep takes me.

[Core XK]

Well my real concern was not MPG or money. My concern is what is best for the engine? Will using the manufactor recommended octane (89) pay off in the end? Or am I wasting money filling it up with Plus instead of Regular. I know premium is a no no unless your tuned for it. Which I will not be tuning it anytime soon.

Thankfully I am willing to put out money if it will increase the life of the XK. I am firm on keeping this truck until it dies. Keeping it properly maintained is number 1 on my list.

Sent from my SCH-I535 using Tapatalk 2

[06JeepXKHEMI]

It wont help or hurt you, even if you did tune it. Running cleaner gas and no off brand cheapo **** will help, but running the 89 or 91 really wont help or hurt the life of the engine itself, just take care of it and dont rag on it like I do and it should run forever!

[lekmedm]

The octane you run has to do with engine timing and compression. The hemi is capable of adjusting the timing to some degree on its own, which is why you can run 87 or 89 octane. It doesn't hurt the engine. You will not get any benefit running 91 if you don't install a tune, but it won't hurt it.

[Matt]

87 can be run as long as there's no pinging. Pinging (pre detonation) will damage the engines. Every engine varies, so one person not having pinging issues does not mean you won't have pinging issues. There's also other factors that influence the likelihood of pinging to occur- higher ambient temps, cooling system efficiency, engine load (towing or heavy throttle), etc.

Facts aside here's my opinion:
If your vehicle is running well on 89, and you're not putting a lot if miles on it (ie not much cost savings), stick with 89.