[Factoryhack]

Jeeps have come stock with many different gear ratios and different tire sizes over time. If you have had the opportunity to drive your Jeep in the unmolested state with stock gearing a tire size and are happy with the performance it is easy to decide on the correct gear for you. Run an RPM calculator and put it back into the same RPMs at the same speed as closely as possible. Like so close that you do not have to re-calibrate the speedo.

On a 4.0L I agree that 4:10s are 31s. 4:56s are 33s. 4:88s or 5:13s are 35s. That puts it close to a Jeep that came stock with 3:55s or 3:73s on stock tires. On a 4 cyl I would go higher.

I prefer to have a Jeep that is more fun when pressing on the skinny pedal.


Loading an engine harder in lower RPMs causes more cylinder and piston wear than spinning it a little faster IMO. I get your idea but you have to look at the big picture and cylinder pressures and their effects on piston loading. It is highly debatable for sure. I think that there is a middle ground. This is the very reason why some strokers in a round about way have short life expectancies.


I KNOW that if I drove your Jeep I would say it has NO giddy up and go. 33s on 3:07 gears would be a complete pig to me. It just would. I have driven a lot of Jeeps as a Jeep mechanic. Some are way more fun than others. Your gearing and tire size is no fun. Your gearing and tire size are for sure hurting your performance and your fuel economy.

You are only using 4 gears. It hurts your economy. The easiest way for me to say this in a few words is that vehicles went from 3 and 4 speeds to 5 speeds with OD. The onto 6, 8, 9 and even 10 speeds with OD. The point is, the closer the gear ratios of the trans, the more gears that it has, and the more time you spend in the optimal power band, the better.



It is all subjective and there are going to be numerous opinions.

I know that I said all of this. My new to me JKU has 3:21 gears. That number says pig all over it. It performs well. I am happy with it. In fact, I LOVE IT!

As far as driving a brick on the highway at 80ish plus, you are not going to get good economy. It is not aerodynamic at all and the wind drag on a vehicle goes up exponentially over about 50 MPH no matter what you drive. I climbs more exponentially in stuff that has flat glass in the front. (JEEPS)

We also have to consider the differential ratio is only part of the story as the other factor is what gearing is available with the transmisssion. If you’re running a ZF 8 speed, a 3.21 ring gear will have a phenomenal launch due to the low gearing in the trans.

I remember when we switched from 545RE to the ZF and learned that a ZF 8 speed with a 3.21 differential has a much lower effective ratio than the old 5 speed with a 3.91 diffy.

 

[RockWoRM]

Generally speaking 4.10 gears are used for 31's, 4.56 gears are used for 33", 4.88 for 35's, 5.13 for 37's, etc.

Actually having done both… gearing to 4.56 w/33s, and then moving up to 35,s, the 4.56 was way low for normal driving. Nice in low range but too low for street. Never used 1st gear, took off in2nd all the time. When I moved up to 35s, it was a perfect combo IMHO. Could still use fifth too. I then added a 4-1 transfer case kit and it stayed that way up until I sold it.
My current Jeep is geared 5.13 and yes, it works well with 37’s. But then again, my six speed has a lower first gear.

 

[Awesome]

Suzie,

I have personal experience.

I ran 31" tires on 3.07:1 ratio diffs for years and years. I THOUGHT that I had 3.73 gears. I only found out I was wrong when I went to regear. I drove on the freeways for years like that, and even towed a boat. Going uphill required downshifting a lot, and if I had a headwind, I could not use 5th gear at all, even on flat ground. Towing uphill meant that 3rd gear was the highest gear I could use.

I then regeared to 4.11:1 gears. It was a massive difference, and I was zooming around in 5th gear everywhere. However, it was geared kind of low for city driving. 1st gear felt almost useless because I needed to shift by the time I was halfway across an intersection.

When I switched to 33" tires, my gearing finally felt perfect. I can use 5th gear on flat ground unless I have a terrible headwind, 4th gear gets me up pretty steep hills at 65-70mph, and driving around town is comfortable.

If you have a 4 cylinder engine you'll probably want deeper gears. But 4.11:1 is just about perfect for 33" tires. If you have an automatic then 3.73 is probably about where you want to be.

 

[fishadventure]

4.56 was way low for normal driving. Nice in low range but too low for street

When I switched to 33" tires, my gearing finally felt perfect

We are all just getting old.
The younger crowd is used to drive-by-wire and think of the skinny like an on-and-off switch or something.

There’s also a dichotomy of driving. Jay Leno gets it: he often extols the virtues of sub-horsepower cars (by today’s standards) and how much fun they are to drive. Or thinking about a Beetle that is a ball to drive at 100HP, or how much joy the ‘53 Corvette was and then they dumped a 265 in it for ‘55. Or how about our jeeps? At 65 mph going up the long inclines Vermont has everywhere I get joy seeing the tachometer floating in the max torque zone. I might floor the gas to no effect, but it just pulls holding speed as if it had more horsepower. I had enough engine roaring rpm when it was 2.5l.

Expectations: I learned to drive when I was 11 or 12 on my dad’s 1960 Ford 300six pickup, three on the tree. I knew what the gears were doing, I listened to the motor, I felt its ease or struggle and adjusted my driving accordingly.

The conversation about 4.56:1 R&P has me in two places in my head:

• do what you want to. It’s yours.
• why the heck do you think you need to feel instant throttle response all the time?

That’s the essence of the dichotomy of driving: what goes on between your ears. That’s also how Wendell Scott was able to compete mid-level and above on junkyard parts; he got it done in his head so well that he didn’t think about what he had, he just created his best and drove it well beyond its limits. Early days of John Force, and Mark Martin, and Lake Speed, et al; they had sub-par equipment and got outstanding results.

So if you think you need 4.56:1 I guess you need it. But having towed without exaggeration tens of thousands of miles in a jeep first with a 2.5 and now the 4.0 I’m happy with driving 4.10:1. I did have trouble towing once, maxing in 4th, but that was with the 56” top and bottom toolbox, a boatload of tools, a couple welders, shop press, etc. 140 miles up I91. 3000# maybe? Known 2000# stuff tows fine and though quite noticeable it doesn’t feel ‘heavy.’
I use my jeep like a jeep, like a truck, it’s my DD “car,” and it’s currently my only transportation. To date I have not experienced any threats to my masculinity from running 4.10s, shifting to 4th when necessary. Within its limits my jeep never disappoints me.

DICHOTOMY ​

 

[Factoryhack]

We are all just getting old.
The younger crowd is used to drive-by-wire and think of the skinny like an on-and-off switch or something.

There’s also a dichotomy of driving. Jay Leno gets it: he often extols the virtues of sub-horsepower cars (by today’s standards) and how much fun they are to drive. Or thinking about a Beetle that is a ball to drive at 100HP, or how much joy the ‘53 Corvette was and then they dumped a 265 in it for ‘55. Or how about our jeeps? At 65 mph going up the long inclines Vermont has everywhere I get joy seeing the tachometer floating in the max torque zone. I might floor the gas to no effect, but it just pulls holding speed as if it had more horsepower. I had enough engine roaring rpm when it was 2.5l.

Expectations: I learned to drive when I was 11 or 12 on my dad’s 1960 Ford 300six pickup, three on the tree. I knew what the gears were doing, I listened to the motor, I felt its ease or struggle and adjusted my driving accordingly.

The conversation about 4.56:1 R&P has me in two places in my head:

• do what you want to. It’s yours.
• why the heck do you think you need to feel instant throttle response all the time?

That’s the essence of the dichotomy of driving: what goes on between your ears. That’s also how Wendell Scott was able to compete mid-level and above on junkyard parts; he got it done in his head so well that he didn’t think about what he had, he just created his best and drove it well beyond its limits. Early days of John Force, and Mark Martin, and Lake Speed, et al; they had sub-par equipment and got outstanding results.

So if you think you need 4.56:1 I guess you need it. But having towed without exaggeration tens of thousands of miles in a jeep first with a 2.5 and now the 4.0 I’m happy with driving 4.10:1. I did have trouble towing once, maxing in 4th, but that was with the 56” top and bottom toolbox, a boatload of tools, a couple welders, shop press, etc. 140 miles up I91. 3000# maybe? Known 2000# stuff tows fine and though quite noticeable it doesn’t feel ‘heavy.’
I use my jeep like a jeep, like a truck, it’s my DD “car,” and it’s currently my only transportation. To date I have not experienced any threats to my masculinity from running 4.10s, shifting to 4th when necessary. Within its limits my jeep never disappoints me.

DICHOTOMY ​

Well, damn. That’s a Walt Whitman level explanation of why we love these old Jeeps.

So many get caught up chasing whatever the new thing is, they can’t comprehend why 500 HP and a bunch of screens isn’t necessary to get the job done. In fact, those things can detract from the driving experience more than they help.

I always think back to a few years ago when I finally got our XJ back after it had been passed on to other family members for 20 years. The first time I sat in it, it was like an old friend. I looked over my shoulder to back up and realized I didn’t need a damn camera because the sight lines were incredibly good compared to any modern vehicle.

As we’ve made “progress”, we’ve also lost some things along the way.

 

[fishadventure]

That’s a Walt Whitman level explanation of why we love these old Jeeps

Ya, sorry. That’s sortofa compliment so thanks.

I do have a problem with writing and it just keeps getting worse.

As we’ve made “progress”, we’ve also lost some things along the way.

I’m trying to think of something intelligent to say here.
But I can’t think of anything other than development, that is development of existing systems, well there really hasn’t been anything new developed since maybe 2002… There’s been a couple ideas in psychology, and a bunch of stuff in neurology, but even the experimental covid vax dates to back then and prior. Even EVs, cellphones, and DNA are just developments on past knowledge. The really only new things are cultural: the mainstreaming of far-left perspectives and the normalizing of aberrational behaviors.

 

[Factoryhack]

Ya, sorry. That’s sortofa compliment so thanks.

I do have a problem with writing and it just keeps getting worse.

I’m trying to think of something intelligent to say here.
But I can’t think of anything other than development, that is development of existing systems, well there really hasn’t been anything new developed since maybe 2002… There’s been a couple ideas in psychology, and a bunch of stuff in neurology, but even the experimental covid vax dates to back then and prior. Even EVs, cellphones, and DNA are just developments on past knowledge. The really only new things are cultural: the mainstreaming of far-left perspectives and the normalizing of aberrational behaviors.

I most definitely meant that as a compliment.

Writing that conveys an idea clearly is rare, so from someone who also aspires to write well, I appreciate it.

As for technological progress, particularly with automotive stuff, it seems as though we keep adding features just because “we can” and rarely taking a step back to ask if “we should”.

With all the normalizing of abhorrent behavior, I’m reminded of Isaiah 5:20:

“They say that what is right is wrong and what is wrong is right; that black is white and white is black; bitter is sweet and sweet is bitter.”

I believe Isaiah was telling us to get our **** together before it’s too late.

 

[Waternut]

So a lot of different ideas out there but the one thing that really matters is the engines potential power. For this sake of this example, I'm going to ignore crankshaft power vs power to the tires. Sure the 4.0 technically makes 190hp but it can only make that at one rpm and that rpm is around 4800rpms. At 3500rpms, the engine can never achieve 190hp in it's current form. Instead the max power you can put out at 3500rpms is more like 155hp and the max power you can make at 2500rpms is around 100hp. The horsepower required to overcome the drag at 75mph is quite a bit more for a Jeep than the horsepower to overcome the drag at 45mph. So as an example, let's pretend you need 70hp to maintain 75mph on flat ground and you need an extra 30hp to get up the steep hill at 75mph. The only way to get that power is to be in a gear that can give you at least 2500rpms and you'd need 100% throttle to do that. 2nd gear won't get you to that speed and 3rd gear will get you to that rpm but it will likely exceed the rpm range due to the large gear ratio difference in the transmission. At this point you have a gear that can't reach 75mph and a gear that can reach 85mph but you're on the rev limiter. 4th and 5th gear won't let the rpms get high enough to give you the power to maintain speed so you need higher numerical axle gears to allow yourself more than one gear option.

I didn't want to write a huge book so hopefully that makes some sense.

 

[whodatschrome]

swap in 2.73 gearing and don’t bother with both 4th and 5th.

 

[fishadventure]

hopefully that makes some sense.

That’s perfect sense.

 

[suzieque]

The input and responses without rants and flaming is what makes these forums invaluable compared to Facebook.
I should have mentioned in my original post that my YJ is mostly city/highway and forestry road driven, i don’t plan on rock/mudding it.
Waternut’s response probably answered my original question the best. I also agree that 3.73 will likely bring my YJ to same RPM range with the larger 33” tires, though as someone else pointed out the 33” are much heavier than stock wheels. I’m hoping to find some 3.73 differentials from a 4 cylinder YJ, but I haven’t found any locally.

 

[HandOverFist]

Ultimately you have to decide if the benefit is worth the cost in the long run. Judging from your stated usage it would seem more of a want than an actual need.

 

[fishadventure]

hoping to find some 3.73 differentials from a 4 cylinder YJ

I think you’d be more than happy with 4.10:1
Plentiful, inexpensive.

 

[Boojo35]

3:73 diffs were in 4 cyl YJs with autos. 4:10s were in 4 cyl YJs with manuals.

 

[suzieque]

3:73 diffs were in 4 cyl YJs with autos. 4:10s were in 4 cyl YJs with manuals.

Thanks, I didn’t know 4:10s came in YJs too.