i will not drop it because of the **** poor info being said.

you have a knock sensor on yours. so if running premium makes your motor run better or get better MPG then you have something going WRONG. if it is pinging on 87 then you need to spend some time under the hood rather than dumping premium in as a bandaid.

the 4.0 was designed to run on 87, its power and economy are best with 87.

running premium will acutually REDUCE power and economy by a small amount because the higher octane actually has a lower charge density than running 87. (Lower charge density = less power and more work for the engine to do the same job, and that turns into more fuel being used causing poorer MPG's)

there is absolutely no reason to run anything besides 87 on a jeep 4.0. even trying to use the cleaning additives of premium as an excuse doesnt work anymore, because most higher quality gas station use these additives in all of there grades.

but go ahead, WASTE your money running premium. OR, do some research and find out how wrong you are.

 

I agree with N20. plus there is so many BS claims of gas mileage from people who have not corrected the speedometer...

 

I have not corrected the speedo because I don't think it's necesary' I've driven with a gps enough to know what speed I'm really going. And I can multiply my odometer by 1.1191 to get the correct reading.
Also the ONE time that I used premium it was 93oct non-oxygenated gas. My jeep ran GREAT! Other than that I run 89 because I really do get better mileage with it. Even with you saying it doesn't it does. 2 identical rifles can shoot the same identical load differently. But if your so right then why get so bothered by other peoples' results?

 

Also my XJ runs great on 87. I just like the results of 89 better, thats all, personal preference.
Again does anyone on here run 31s with 4.10s? I'm really curious about this set up.

 

I had 4.88s on 33s that measured to be like 32" Muds. And it was sweet. Had plenty of power and wasn't too bad on highway. Of course a month later I put the 35" MTZ on and its still awesome. Plenty of power good RPM band on highway.

4.88s on a Dana 30 worrys me a little. the 4.88 pinion gear is small and I was going to do 4.56 but I bought the Dana 30 with the gears in it. But 4.56-4.88 isn't much different. Its not worth laying out the dough for 4.10s cuz its not far from 3.55s. Do it right the first time.

Oh if you all are so worried about MPGs. Go buy a Honda, Its a Jeep what do you expect. I get 10 mpgs with 4.88s and 35s. My honda gets 30

 

I would agree, IMP its not worth re-gearing unless your going 33"+

 

Don't mean to barge in here but I gotta tell ya. I recently had 4.56 gears installed with 32" km2's on my AX-15 equipped xj. My rpm's now hover around 2,800-2,900 @ 70 mph. I'm lucky to get 250 miles per tank driving like an old lady. Gonna have to scout some local junk yards for some 3.73 or 4.10 equipped axles to get some mileage back. Coming from 31's and 3.07 gears, it's a huge difference. Hope this helps.

 

i was told the 4.10's were stock for 33's so......what do we do?

 

Don't know what the deal is, if your speedometer is off or if your 32 inch tires are really shorter, but you should be doing about 2650 rpm at 70 mph not the mentioned 2800-2900 at 70.

 

My wifes wrangler has 35's and 456's with a 3 speed auto and it has the right speedo gear and it runs 2900 to 3k at 72ish mph. You have a 5 speed it should not be that high unless you do not have speedo correct.

 

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35's, 4.56's, and an AW4 trans in OD, At 65mph I run about 2350 RPM according to my scan tool and my speedo agrees with a 39 tooth gear. double checked it last weekend...

 

Yea i wish it had the 4 speed od but oh well its not a DD so it works for us. It does great on the trail it acts almost like the t-case has a 4 to 1. I would like it to run around the 2500 but unless i sell it and buy a newer one with the aw4 then this is what i have to deal with i dont run much past 70 anyway on this one.

 

If you had 3.55 to start yes. 3.07s will level out at 3.55 with stocktires being 27" or 3.73 with 28"s.
Find your original set up and look for other set ups with the same speedo tooth count. My dad has 33s on 3.55s, don't get 3.55s. On windy days he has to drive in 3rd gear. Hope this helps

http://www.xjtalk.com/showthread.php?t=4398

 

it also depends on what transmission you have.

 

I believe that the gears in mine are stock. I have 33s and I think it's great just the way it is . It gets great mpg on the hwy , isn't too bad in the city and has plenty of power too .At what the speedo says 65 mph ( really like 78 ) it spins 2-2,100 rpm . 4.0L auto