Where should RPM's be?

I have seen all the calculators for getting the proper gears but they really seem to be rated on what RPM you want for a given speed. I have not had my XJ long and it came with 35's. I have a d44s I am about to start prepping for a swap. Problem is, I do not know where cruise RPms should be. I have a 2000 with 3.54's in it know and an overdrive that Hunts for between constantly. This is more a Daily Driver than a Weekend runner. I like it so much I drive it all the time. I have gotten some many different opinions on the gears. I am thinkink 4.56's but would really like the opinion of this board. I know most of you have tried all of things and can give me the best advice.

So, what RPMS should I be running at 65 mph? Low 2000's?
Are ARB's the best selectable Lockers? Is there a good place To get them?

I have been reading alot of articles and can not find where the 4.0 really "makes" power. Any advice would be greatly appreciated.

My 92 MJ with HO 4.0, AW4, and stock sized tires runs like this;

(in overdrive)
55......2100 rpm
65......2400 rpm
75......2850 rpm

As mentioned before by people FAR smarter than I, the 4.0 is a short square motor. That translates to a higher, safe rpm range than compared to a v-8.

With stock tire size, the AW4, and 4WD, you should be just under 2100 at 65 m.p.h. with 3.54s. Here's a great place to check this stuff. http://www.dfw-tx4wd.org/Tech-Dept/Tech-Dept/gears.html

XJ I6's really like 2600-2800 cruising.

Depending on which tranny you have I like 5.13's and 5.38's (mine) for 35's.

ARB's are good selectable lockers as well as OX's (if they are currently available). I haven't heard if the electrics have gotten the reliability issues worked out yet. Take your pick on a place to get the ARB, almost everyone sells them.

MooseXJ said:

what? that sure seems really low for 35s, i have 37s and 4.88s and they seem perfect.

Click to expand...

Have you ran lower?

With the .705:1 od in my old AW4 the 70 mph revs are 2500-2600 and it likes to cruise there all day. 80 revs are still below 3K but with 250,000 miles it's earned it's non daily driver status. Mileage is around 18 mixed trail and street.

Why would I want to go any higher?

xjcrawlr said:

My 92 MJ with HO 4.0, AW4, and stock sized tires runs like this;

(in overdrive)
55......2100 rpm
65......2400 rpm
75......2850 rpm

As mentioned before by people FAR smarter than I, the 4.0 is a short square motor. That translates to a higher, safe rpm range than compared to a v-8.

Click to expand...

Huh?

Auto tranny, stock 225/75R15 tires, stock 3.54 gears, in overdrive:
55 .... 1777 RPM
60 .... 1938 RPM
65 .... 2100 RPM
70 .... 2260 RPM

As to the engine, yes the 4.0L is an over-square or short-stroke engine (meaning the bore diameter is greater than the stroke) -- but so is just about every V-8 that has been offered by an American manufacturer since the early 1950s. There is no way that a 4.0L has a higher safe rev limit than a V-8.

So after looking at the websites I think that 4.88's would be what I need. I tow a small 5x8 trailer with a mower on it and a 14 foot jon boat from time to time. This is a daily driver with only 105,000 on it.

What you think? 4.88's should get me the power and decent mileage. At 65 I should be around high 2200's - 2300 RPM.

i would do 5.13s but i dont do very much 70+ driving. 4.88s sound good. i've got 4.56s and 33s and already know i wont be happy when i go to 35s

I'm offering up another vote for 5.13.

I threw a set of 4.56 gears in mine last year while I still had 31's on. I was planning on getting rid of the 31's soon and getting 33's or even 35's at the time. I still have the 31's on and absolutely love it. My mileage actually went UP a couple MPG. Now that the 31's are getting pretty worn and I'll have to replace them soon, I'm looking at a set of 5.38 gears for my future 35's. I can't imagine I ever will, but if I later want to go to 36's or even 37's, my gearing will still be great.

Oh, and with the 4.56 gears and 31's I spin about 3000rpms at 80mph. I usually keep it at 75mph spinning around 2800rpms.

Being that I have absolutely no experiance I'm going to chime in here.

5.13's at the highest (by highest I mean ratio wise; non numeric).

Why?

1: Do you really what to drive a lifted XJ on 35's any faster than 75? 5.13's will put the engine in a sweet spot at 75.

2: Lower gears are nicer on the trail of course.

3: If you're gonna tow anything choose some lower gears.


Just so you see where I'm coming from I intend to go 4.88:1 for my gearing change. That'll be with true 33" tires.

My old Renix XJ loves to cruise at 2400 - 2800 all day long. I could push it to 3000 - 3200 if necessary (I mean push it to spin that fast for two hours on end while climbing mountains in third gear).

The power band is your friend.

Sequoia