Re-gearing question

[AZJeff]

BACGROUND:  My MJ is equipped with an engine & control system from a 2000 XJ, and is thus OBD2.   It also has a D44 in the back and a low-pinion D30 up front, with 32” tires.  The current axle ratios are 3.55:1

Out here in AZ, when I am driving in the mountains on the highway (65 mph), the trans keeps shifting down as soon as the slightest grade.  I end up having to lock it out of overdrive to avoid this annoying “hunting” for the best gear.

 

I am thinking a rehearing of the axles would help to solve this.  The question is:  what would be the best ratio to install?

 

BTW, I am not looking to make a rock crawler here, so I am looking for the lowest numerical ration I can install that will get rid of the struggle to stay in high gear on the highway.

 

[Pete M]

have you run your numbers through Eagle's tire-vs-ratio chart? 

 

[Smokeyyank]

4.10s but 4.56s would be my pick. I ran 4.10s on my XJ with 33s and it was fine, but always wish I had deeper gears off road and going up the mountain. Around town 4.10s was plenty. 

[acfortier]

That chart is great, but in general, what should we be looking for? What RPMs should we be aiming for while going 65?

[75sv1]

Gear Ratio Calculator (grimmjeeper.com)

 

That's the Gear Ratio chart I use. I do have an XJ on 32-33"s. I have 4.10's and run in WV. I'd say at 70 mpg 2250 rpms. At 55 rpms, I'm about 2000 rpms. I do feel a bit sluggish with 4.10 gears and 32.5" tires.  Some of this is a power issue. When I was on 31's and 4.10's, I still had issues on some inclines. I did have the 99+ horse shoe intake and a 2.5 exhaust and header. I still had to downshift. I bores out the TB to 60mm. Then it was good. When I went to 32.5" tires, it still does OK. But it does seem sluggish. I did plan to go to 4.30 gears. Still, I want to see what a stroker motor will do with this combo. 

    Also, some of the passes out West, are well steep. I'm thinking of Raton pass on the CO, NM border. I would be tempted with 4.56 gears. It might buzz a bit.

[derf]

I had 4.56 gears with 265/75R16 tires (32s) in my 2001 XJ with a 4.0 and automatic.  It was just about perfect for higher altitudes (Denver area) and highway cruising was good.  No gear hunting happened.  Went all over Moab (lower altitude) and never thought I needed more or less axle gear.

 

With a manual I might consider 4.10s over the 4.56.

 

It's important for the automatic that you have the correct speedometer gear in the sender in the transfer case.  It affects shifting with the automatic I believe.

 

[Pete M]

If I found some 4.10 axles in a junkyard, I'd run them because I'd be saving a bunch of money.  but if I was paying for all new stuff, 4.56.  with no hesitation.   

 

 

[AZJeff]

9 hours ago, Pete M said:

have you run your numbers through Eagle's tire-vs-ratio chart? 

 

Five minutes after I made my post, I saw Eagle’s chart.  I drive about 85% in town, and the rest on mountain grades at highway speeds.  4:56 seems a bit overkill for occasional use, but convince me I am wrong.

[ghetdjc320]

4.56 for sure if your getting new gears. The auto tranny is a power hog. An ax15 might be fine with 4.10

[Pete M]

3 minutes ago, AZJeff said:

Five minutes after I made my post, I saw Eagle’s chart.  I drive about 85% in town, and the rest on mountain grades at highway speeds.  4:56 seems a bit overkill for occasional use, but convince me I am wrong.

 

 

I would do 4.56 for sure.  the extra power is nice for stop and go and for climbing hills.    don't fear the RPMs.  the MPG math is more complicated than straight up counting engine rotations.   

[DesertRat1991]

Go with 4.28 or 4.56

[Smokeyyank]

12 hours ago, AZJeff said:

4:56 seems a bit overkill for occasional use, but convince me I am wrong.

 

I mean no one has said 4.10s would be ideal...... Plus who doesn't want a little extra pep in your pedal when you hit it!  

 

If you have a manual, yes, you can get away with 4.10's because the extra gear. With the AW4 you should get a little deeper gear to help. Not 100% positive so someone correct me if I'm wrong, but the torque curve on the 4.0 picks up about 2300-2600 so you should trying to stay around that range. 

[Pete M]

4 minutes ago, Smokeyyank said:

 

If you have a manual, yes, you can get away with 4.10's because the extra gear. With the AW4 you should get a little deeper gear to help. Not 100% positive so someone correct me if I'm wrong, but the torque curve on the 4.0 picks up about 2300-2600 so you should trying to stay around that range. 

 

yup, factory autos have 3.55s vs manuals with 3.07.  they need a bit more gear.  

[Tex06]

My $0.02, it comes for free so take it as such:

 

The AW4 has an overdrive ratio of .75 or .79 depending on what year it was pulled from. If you're happy with everything else about how the jeep drives, and just want to eliminate hunting at 65mph, the 4.10 gears should do you fine on 32" tires. The AX-15 has a .79 OD ratio so the 2 may have the same gearing at your "problem" speed, number of gears doesn't matter then.

Full disclosure, I run an AX-15 on 32" tires and have 3.07 gears in my Comanche. My last XJ had an AW4 and 33" mud tires on 3.55 axles and ran 65 mph (GPS calculated) without hunting but it was in the North Georgia Mountains so it down shifted when climbing and hit OD on the flats consistently. 

[75sv1]

I'd drive around with your present setup in 3rd. See if the noise is acceptable. Also, 4.56 might lose you 1/2 mpg to 1 mpg. 

[derf]

The factory setup with 3.55s and 205-225 series tires was a compromise to get more fuel economy at the cost of driveability.

 

Going to 32's, you'd want to run 4.10s.  That would get you back close to stock type performance.

 

The 4.0 runs best with 4.56 gears and 32" tires.  It puts the highway RPMs up where they need to be to pull hills without down shifting while at the same time not running too high so they don't kill your fuel mileage.

 

Personally, you'd have to fight to convince me to run anything but 4.56 with 32s, especially after I ran that combo for a few years in an XJ that I recently sold.

[Eagle]

The stock setup with auto (225/75-15 tires with 3.55 gears) turns 2100 RPM at 65 MPH in over drive. 31x10.50-15s with 3.73 gears turn 2085 RPM at 65 MPH.

 

In other words, 31" tires with 3.73 gears are effectively exactly the same as a stock automatic. My rule of thumb is that 3.73 gears are about perfect for 30" tires, 4.10s are good for 31" tires, and for anything bigger than 31s you need at least 4.56 gears. If you run a Dana 30 front axle, last I knew the steepest gearing you could buy was 4.88, and those have such a small pinion that's it's rather weak setup. If you use one of the "Dana 44" front axles out of a TJ Rubicon, the bigger pumpkin will allow you to get into deeper gearing -- but it's a low pinion axle.

 

31x10.50-15s with 4.10 gearing turn 2292 (call it 2300) RPM at 65 MPH. That's not bad for highway work. My frame of reference, for those of you worrying about "burning up" the engine, is the AMC cars these engines were originally designed for. My family was an AMC family. We owned just about every early variation of the engine: 199 c.i.d, 232 c.i.d, and 258 c.i.d. They didn't have over drive in those days, and the models with standard transmission were geared to run 24 MPH per 1,000 RPM. That means 60 MPH was 2500 RPM, and 3000 RPM was a 72 MPH cruise. And the engines easily went well over 200,000 miles.

 

Jeff, a bit more crunching shows me that in overdrive you are turning 1904 RPM at 65 MPH. That's actually below the torque peak for the engine, so it's not a surprise it won't pull long upgrades on the highway. When you lock it out of overdrive, in third gear you're turning 2539 RPM. 4.10 gears will get you to 2205 RPM, which kind of splits the difference. But the torque peak for the 2000 XJ engine is at 3,000 RPM, so you'd still be well below the torque peak. 4.56 gears would get you closer -- 2453 RPM. But that's very close to what you're turning when you just lock it in third gear, so I don't know if you want to live there all the time or not.

 

[ghetdjc320]

Good to factor in the parasitic loss in the auto tranny also as well as the increased rolling mass of those 32’s. I still hunt for gears with a pepped up 4.0 and 4.56’s on 33’s with the aw4. But that’s what the transgo shift kit is for 

[AZJeff]

15 hours ago, ghetdjc320 said:

Good to factor in the parasitic loss in the auto tranny also as well as the increased rolling mass of those 32’s. I still hunt for gears with a pepped up 4.0 and 4.56’s on 33’s with the aw4. But that’s what the transgo shift kit is for 

While we are on this topic, tell me about the Transgo shift kit.

[ghetdjc320]

4 hours ago, AZJeff said:

While we are on this topic, tell me about the Transgo shift kit.

It’s a reprogramming kit that adjusts shift points as well as provides for a true 1st gear hood when in 1-2. Downshifts much quicker and doesn’t hunt through gears as much. The single kit comes with a couple of different installation options depending on how agressive or smooth you want your shifts to be. NAXJA also has some good write ups on the kit. 

[cruiser54]

I like the "leave it in 3rd gear" option. Give it a try. Free.