Your output gearing will be lower as 3.73 geared JK will be using a lower transmission gear than a 4.88 geared JK. This is what makes up for taller diff gears. Here's what it looks like:

JK 3.73 gears
1st gear = 16.64
2nd gear = 9.74
3rd gear = 6.42
4th gear = 4.66
5th gear = 3.73
6th gear = N/A

JK 4.88 gears
2nd gear = 12.74
3rd gear = 8.39
4th gear = 6.1
5th gear = 4.88
6th gear = 4.10

Once they reach about 65-70mph, 3.73 JK will be in 3rd gear, and the other one in 4th, and the 3.73 JK will be running slightly higher RPM. This is why I'm saying it should be very close. In anyway, I doubt many people regear just to race their JK, so this comparison isn't that practical.

 

Re gear and add lockers if you dont have them already.....

 

I had 4.88 in a 2 door manual with 35's. If you are just drive around town you are good but if you hwy drive then be prepared to be sitting close to 3k rpms at 65-70mph, mpgs bad. if i cruised at 60-65 then i could get between 16-17mpg and that is with a ripp supercharger and longtube headers. Off road it is great, crawl for days. If i had to do it again i would have opted for 4.56's. but not many mountains around here just rolling hills.

 

Another thing to keep in mind is what your actual tire size is. When you run what a manufacturer calls a "35" inch tire at 30 psi a lot of them are closer to 34". At 34 and 4.88 you are running 70mph at 2837 rpm. Still great for crawling and more than acceptable highway but not optimum mpg. Depends on what you value more. Climbing or fuel mileage.

Tire tread also greatly affects fuel mileage. A loud MT will more likely have greater rolling resistance than a quiet AT.

 

Hi guys

I think there is a lot more to re gearing than the 70mph table that keeps appearing. For me anyway.

I have a 2012 manual with 3.21 gears, with BFG 33" AT tires.
I use it 90% for dune driving here in Dubai.

For ALL the technical little dunes and climbing big dunes, I use 1st gear and between 3000 and 6000 rpm to keep in the power band of the motor. For the 'crazy stuff' you sit at 5500 to 6000 rpm for a short time for max performance. It is a power thing and a tire speed over the soft sand. Any slower and you tend to sink in and not ride over the sand.

2nd gear is only OK when you don't need full power, as at 'Dubai dune speeds' you will only have 1500 to 3000 rpm. 2nd and 3rd are great when you get onto flattish ground or a dirt track, where you can keep speed and don't need full power.

6th gear is totally useless on the highway....

I have been 'running the numbers' using the GrimJeeper web site.

4.10 is rubbish, for me. 1st is too slow and 2nd is way to fast.

4.56 initially looked good. 6th would give about 2700rpm on the highway. The problem is at my 'Dubai dune speeds' I would need 2nd gear and lower rpm or higher speed than 1st gear/3.21. The problem is, if I loose speed/rpm, I would need to slow really slow before 1st gear would be usable. This would drop right out of the power band before being able to drop to 1st. Not good 1/2 way up a big dune or worse still, along the side of a steep soft dune.

4.88 looks much better. 1st (and reverse) would be like a slight low ratio. Good for moving off on a steep hill or tight spot where I tend to be sliding the clutch on my 1st/3.21 setup. 2nd is just slightly lower rpm or high speed than 1st on 3.21. So for the 'crazy stuff' I would only be at 5000 to 5500 rpm.... Or I could drive a little faster.
6th would give just under 3000rpm for 70 mph. It's a little high, but to get the other gears where I want, I have to accept this.

Now all I need is some $$$$ for gears and lockers!!!!

Regards
Paul

 

Adding 1000 or more RPMs to maintain highway speeds as opposed to only a couple hundred is the difference. Sure, he could probably do 65-70 in second gear, but no one does that because it's stupid. You add strength to your bottom end and return your drive line to essentially a more natural setting for efficiency because of the weight that you have added. Why don't you run a mile, then run the same mile with ten pound weights on your ankles and wrists, then add a 30 pound weight vest and see how much more inefficient you really are. The same concept applies with the mechanical leverage in your gears. You are dismissing this fact and saying that it is simply easier for your engine to accommodate the extra weight by sprinting. In other words you would suggest foregoing additional mechanical leverage in favor of higher output by your engine. This, sir, over time will undoubtedly lead to great additional wear and tear on your engine, and transmission.

You are right about cost, it isn't cheap. But chances are you can find someone who does them for a living that is a part of a local off road club. I did that, learned how to do it from that guy, and it saved me a thousand bucks and I learned a whole hell of a lot about axles in the process. As for the pinion gear, you offset this by keeping your stock axles the weak point. It is easy to replace an axle shaft of the trail. That generally negates your breakage issue. However, most reputable aftermarket gear manufacturers only use chromolly or higher grade alloys for their tensile strength. From a pure tensile point of view, an after market pinion will be stronger than a stock pinion, despite the loss of teeth. However, I have never seen any person strip a ring and pinion who wasn't in a bind, and didn't just floor it. In other words, inexperience and an adverse situation caused the failure. Not the actual loss in teeth. In other words, it would have happened regardless of gearing in the bottom end.

In sum, gears allow you to maintain mechanical leverage so that you can maintain your momentum without downshifting as often or as far. The pinion gear (especially in this particular case) will not be weakened to a point of concern. Your axle shafts are, and will remain the weak point in your drive line. Lastly, yes it is costly, but the advantages far outweigh the disadvantages, and furthermore you could probably do it yourself with the right tools in place and a person who has the previous knowledge.

If the OP really wants to solve this issue though, go find someone with a manual jeep with 4.88's, drive their jeep, and feel the difference for yourself.

 

What are you talking about? I can do that all day long in 3rd gear auto. With a manual that should be easily done in 5th (maybe 4th if you got 3.21 gears). What 2nd gear are you talking about? Your RPM that way will be around the same area as if you regeared. Mine are currently at RPM of 5.38 gears.

It seems like you're totally missing the subject. Regearing makes your engine "sprint". Not regearing and using a lower transmission gear makes your engine "sprint". What are you arguing against, exactly?!

And, what if I want to upgrade my shafts, 'cause say I keep bending stock ones or want to install a locker? Just keep replacing stock shafts, always carry a couple spares, and hope when the shaft goes it doesn't take out the carrier, and/or the housing with it, etc?

And where did you get this information from?

Just keep believing and hoping. I don't feel like explaining the obvious.

I've already said that now I'm shifting a whole lot less. 1 gear on highway. 1 cruising gear in the city, plus 1 starting gear. If I need to go, all I do is just lightly push the gas pedal, no need to downshift. There are gobs of power available because my cruising speed is always in the 2000's.

No one here ever said that there won't be any difference. Please reread the whole thread and what has been said.

 

So instead of shifting 3 times your only shifting 2, BUT it's an auto... Stop trying to compare it to a stick. Also what works for you doesn't necessarily work for others. Reading this tread the only impression I get is your trying to justify why you won't/ don't regear.

Also, going off topic a bit, how are you getting 19 mpg obviously you don't have OD so I find it hard to believe that your getting better then most get stock?

 

One thing that hasnt been covered was the effect of larger tires on the transmission. The added weight/ rotational mass of the tires directly impacts tranny operation and utimatly life span. An example of this can be seen when towing a trailer. If you have a tranny temp gauge you will see it rise even with light towing.
Gears aid tranny life because they act as leverage to turn larger tires.

 

Your wasting your time. Terminator doesn't understand how gearing works and tries to justify his use of running his jeep as a 3 speed manual instead of how the auto transmission is designed. Also he compares his rig to a manual and does not understand that 321 or 373 with a 35 in tire is at a disadvantage to that of one geared in 488 etc offroad.