Gears for 37's and auto

[conFUcius]

It all depends on driving style. where I live, 85mph is average on the freeway. The 4-1 transfer case insures plenty of gearing off road. For me personally, freeway speed is the critical component and therefore I’d adjust all of your ratios across the board up one. The 4.56’s on our current XR with stock 35’s are boardline too low for me and it is a 3.6L. If you’re the type of Jeep owner who rarely exceeds 70mph, theres no penalty for gearing too low.

I'd adjust and say if you drive above 80 MPH, maybe shift everything one for freeway driving. I drive 75 MPH just fine on 37s with 5.13s and it's my daily driver in the field for work.

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[grimmjeeper]

I’ve been contemplating gear changes as well. Sounds like everyone is saying for a 2.0 auto on 37s they recommend 4.88.

I tow a 3,000 lb camper so I figured 5.13 may be a better move. Thoughts?

Stepping up one gear does help with towing.

 

[Crowbar]

37s and 4:88 are a very happy couple. They are happy with an auto or Std trans and I have run both. I will say the 5 sp auto is not as comfortable with 37s/4:88s as is the 8-speed auto, not so much as to be an issue but gain better performance from the 8 sp setup.

I point you to forums that UNLIKE THIS ONE(at least so far) lack the HONESTY that I have seen on here.

On other forums bragging on how your 3.55 axle works GREAT with you 40 in tires, gives you 29 mpg and has not affected performance at all. In fact one said he drove across the country and averaged 85 mph and 27 mpg.

I would tell that most Jeep forums a high % of nonowners posing as owners and Jeeps. I have not sensed that in this forum, prob one best forums out there...

You can run 4:10 with 37s, not a happy drive, 4:56 will improve the performance and 488 the happy camper.

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37s and 4:88 are a very happy couple. They are happy with an auto or Std trans and I have run both. I will say the 5 sp auto is not as comfortable with 37s/4:88s as is the 8-speed auto, not so much as to be an issue but gain better performance from the 8 sp setup.

I point you to forums that UNLIKE THIS ONE(at least so far) lack the HONESTY that I have seen on here.

On other forums bragging on how your 3.55 axle works GREAT with you 40 in tires, gives you 29 mpg and has not affected performance at all. In fact one said he drove across the country and averaged 85 mph and 27 mpg.

I would tell that most Jeep forums a high % of nonowners posing as owners and Jeeps. I have not sensed that in this forum, prob one best forums out there...

You can run 4:10 with 37s, not a happy drive, 4:56 will improve the performance and 488 the happy camper.

The 2.0 is not available as a manual? Looking for 2.0 recommends. I have a 3.6 with 5.13s and it is perfect.

 

[Crowbar]

The 8 speed auto allows you to over or under gear with minimal consequences. The 1st gear is so low compared to any auto ever offered in a Jeep it's almost comical. A jk with an auto would require 5.13 gears just to match a stock JL Rubicons 1st gear drive ratio. The double Overdrive does the opposite on the top end. A JK with 4 10's would have the same freeway rpm as a JL with 4.88's. In our 2019 we drove, wheeled, and roadtripped for 37k miles on 37's and stock 4.10s and never felt a need for more gearing. When we went to 38's, I decided to regear. My jump in head first personality got the best of me and we went all the way to 5.38's. Way too low period. It was in 8th gear all the time. At freeway speed it felt like it needed to shift but there was nowhere to go. Destroyed fuel economy, somehow was slower 0-60, and just didn't cruise on the freeway as well. I could see 4.88's but absolutely no lower. With the new XR, I won't even consider regearing from the 4.56's.

So your currently driving a 2.0 with 37's and 4.56?

 

[CarbonSteel]

It all depends on driving style. where I live, 85mph is average on the freeway. The 4-1 transfer case insures plenty of gearing off road. For me personally, freeway speed is the critical component and therefore I’d adjust all of your ratios across the board up one. The 4.56’s on our current XR with stock 35’s are boardline too low for me and it is a 3.6L. If you’re the type of Jeep owner who rarely exceeds 70mph, theres no penalty for gearing too low.

I drive 85 in Wyoming all the time with no issues and the engine is not trying to leap out of the vehicle either due to excessively high RPMs. At 75MPH I am at exactly 2500 RPM and about 2700 at 85MPH which is not even 50% of the RPM range of the 3.6L--BUT you are going to pay for running that fast no matter what your gearing via loss of MPG.

As you said though, it absolutely depends on driving style and what you are either willing to live with or accept. At the altitudes I drive, I am not willing to accept the mundane performance of the 3.6L it has being under-geared.

I have had more gear ratios and tire size combinations than the vast majority on the site and so it is easy to know what does and does not work well with a 3.6L/ZF8 combination.

 

[Crowbar]

I have an 3.6 E torque XR with 4.56 and 37's . Plenty of on road acceleration, Holds 8th gear on the highway. Does fine off-road.

2.0 is a completely different torque curve.

 

[CarbonSteel]

An automatic can compensate somewhat for a lack of low RPM torque. The stall speed of the torque converter makes all the difference.

If you have a high stall converter, the engine will rev up to a higher RPM at low speeds before the Jeep gets moving. This gets it up to a place where it can make more torque. When that happens, the perception is that it's accelerating just fine even with mismatched gears. And it's why so many people claim you don't need to regear.

That being said, I'd agree with your recommended ratios.

Fully agree, but I was not aware that a stock ZF8 had a high stall converter and thus why I said the transmission cannot compensate for loss of torque.

With that said, does the ZF8 have a high stall converter?

 

[grimmjeeper]

Fully agree, but I was not aware that a stock ZF8 had a high stall converter and thus why I said the transmission cannot compensate for loss of torque.

With that said, does the ZF8 have a high stall converter?

They tune the stall speed to the engine. I would bet money there are different converters with different stall speeds between the 2.0 and 3.6.

Likewise, I would bet the 392 and diesel have drastically different stall speeds on the 8HP75.

 

[Zandcwhite]

I drive 85 in Wyoming all the time with no issues and the engine is not trying to leap out of the vehicle either due to excessively high RPMs. At 75MPH I am at exactly 2500 RPM and about 2700 at 85MPH which is not even 50% of the RPM range of the 3.6L--BUT you are going to pay for running that fast no matter what your gearing via loss of MPG.

As you said though, it absolutely depends on driving style and what you are either willing to live with or accept. At the altitudes I drive, I am not willing to accept the mundane performance of the 3.6L it has being under-geared.

I have had more gear ratios and tire size combinations than the vast majority on the site and so it is easy to know what does and does not work well with a 3.6L/ZF8 combination.

You’re paying extra when it’s down hill with a tail wind where I’d be spinning 500 less rpms under minimal load conditions. Having run both over geared and under geared for 10’s of thousands of miles I’d much rather have the ability when the opportunity arises to spin 2200 rpm’s instead of 2700. If I prefer the throttle response, I can put it in 7th and have the same drive ratio you do in 8th. Drafting a big rig, flat roads, down hill with a tail wind, etc are all reasons we have 2 overdrives in the first place. Gearing so low that you cruise in 8th at any speed above 50mph (where I was at with 5.38’s and 38’s) just feels like I’m leaving a lot on the table. The trans has 8 gears, why spend hours at a time never seeing anything but 8th. My driving habits didn’t change and we lost between 2-4mpg vs the same 38’s on 4.10‘s. The decreased fuel efficiency was across the board from daily driving around town to 1,000+ mile road trips. Gone are the CJ days where we had to gear so low for trail use that we couldn’t keep up with the highway traffic for sure, but I don’t see any gain from gearing too low and am much happier towards the other end of the recommended gearing for tire size.

 

[CarbonSteel]

You’re paying extra when it’s down hill with a tail wind where I’d be spinning 500 less rpms under minimal load conditions. Having run both over geared and under geared for 10’s of thousands of miles I’d much rather have the ability when the opportunity arises to spin 2200 rpm’s instead of 2700. If I prefer the throttle response, I can put it in 7th and have the same drive ratio you do in 8th. Drafting a big rig, flat roads, down hill with a tail wind, etc are all reasons we have 2 overdrives in the first place. Gearing so low that you cruise in 8th at any speed above 50mph (where I was at with 5.38’s and 38’s) just feels like I’m leaving a lot on the table. The trans has 8 gears, why spend hours at a time never seeing anything but 8th. My driving habits didn’t change and we lost between 2-4mpg vs the same 38’s on 4.10‘s. The decreased fuel efficiency was across the board from daily driving around town to 1,000+ mile road trips. Gone are the CJ days where we had to gear so low for trail use that we couldn’t keep up with the highway traffic for sure, but I don’t see any gain from gearing too low and am much happier towards the other end of the recommended gearing for tire size.

I guess I am not following how I am paying extra when going down hill or a tail wind--the engine has less of a load in both cases and the MPG will go up accordingly. I know this to be true because I use more gas climbing up to Dillon/Keystone from Colorado Springs and whole lot less on the way back since it is downhill all the way.

To be clear, a JLUR that weighs 5970 is not an econobox by any stretch of the imagination. I average between 17-19 MPG at speeds less than 75MPH and 15-16 at speeds above that and never bought my JLUR to see 25MPG else I would not have bought it. Also, running 2200 at highway speeds with a 3.6L is not optimum either and you will be in 7th at any hint of resistance (winds, hills, mountains) and thus you will be spinning at higher RPMs than I would be in 8th. I only went up about 150RPM by going from 4.88 to 5.13 at highway speeds and that is nothing, but it was enough to make a difference at altitude.

Because I started with 4.10s and went to 4.88 and finally 5.13 instead of straight to 5.38s and 37s, which has given you a bad experience, my experience is vastly different. My Jeep does not feel like it needs another higher gear at highway speeds and it will downshift when the need arises.

With the weight of my JLUR and the altitude it is normally in, my gearing is perfect, but works equally well at sea level. I have also spent thousands of miles with more gear ratios and tire sizes that nearly everyone here and definitely would not have spent the money to re-gear to 5.13s had it not been necessary.

Totally fine with your experiences, they just do not match mine.

 

[Crowbar]

You’re paying extra when it’s down hill with a tail wind where I’d be spinning 500 less rpms under minimal load conditions. Having run both over geared and under geared for 10’s of thousands of miles I’d much rather have the ability when the opportunity arises to spin 2200 rpm’s instead of 2700. If I prefer the throttle response, I can put it in 7th and have the same drive ratio you do in 8th. Drafting a big rig, flat roads, down hill with a tail wind, etc are all reasons we have 2 overdrives in the first place. Gearing so low that you cruise in 8th at any speed above 50mph (where I was at with 5.38’s and 38’s) just feels like I’m leaving a lot on the table. The trans has 8 gears, why spend hours at a time never seeing anything but 8th. My driving habits didn’t change and we lost between 2-4mpg vs the same 38’s on 4.10‘s. The decreased fuel efficiency was across the board from daily driving around town to 1,000+ mile road trips. Gone are the CJ days where we had to gear so low for trail use that we couldn’t keep up with the highway traffic for sure, but I don’t see any gain from gearing too low and am much happier towards the other end of the recommended gearing for tire size.

I had stock 4.10 with 37s for awhile before regearimg. While it works because of the 8 speed it felt sluggish and heavy. I'm also at 4400 feet and desert heat and mountains. 5.13 made it feel 800 lbs lighter and get 17mpg around town and 18plus on trips with steel bumpers, winch, and beadlocks.

 

[XxsullyxX123]

I have a 2020 JLU Rubicon Recon ( so its the 2.0 with the e-torque) 8spd, 4.10s & steel bumpers. I am on 37" nitto Ridge grapplers and aev 2.5" lift now for the last 10,000 miles and it feels fantastic. sure it has slightly less get up and go then the stock 33's , but it is still SOOOO far better than my 3.6 auto JKUR with 3.73's on stock tires. I commute on the highway and see 8th gear all the time. off the line can spin tires (not that i need to), and never feel like i am lacking performance. I made sure to reset the tire size in the ECU to allow the speedo to be correct and shifting points as soon as I upgraded tires. this makes a big difference. thrilled with the combination, and see no reason to regear. I am in NY not too far outside of the city so I am around 500 feet above sea level most of the time and not super hilly. everything factors in!

Seeing 18mpg on average according to the gauge. i haven't calculated myself.

 

[Zandcwhite]

I had stock 4.10 with 37s for awhile before regearimg. While it works because of the 8 speed it felt sluggish and heavy. I'm also at 4400 feet and desert heat and mountains. 5.13 made it feel 800 lbs lighter and get 17mpg around town and 18plus on trips with steel bumpers, winch, and beadlocks.

The 2.0t has a huge advantage at altitude thanks to the forced induction. An NA engine loses 3% of its power for every 1k foot of elevation. A turbo engine loses 1%. At 10k feet that's a 30% loss for the v6 vs just 10% for the turbo. The gearing is more necessary for the v6. Again, I'm not saying 4.10's are optimal for 37's with either engine. Neither are 5.38's optimal for 38's in my experience. I think 4.88's would be the sweet spot for me, but likely won't spend the money to go from 4.56's to 4.88's as the gain is minimal. Personally I'll never gear another Jeep with the 8 speed in the 5's again unless it's on >40's.

 

[kah.mun.rah]

Depends on a lot of factors but I think you will find that the most common, balanced performance and mpg, and strength is 4.88.

 

[conFUcius]

I had stock 4.10 with 37s for awhile before regearimg. While it works because of the 8 speed it felt sluggish and heavy. I'm also at 4400 feet and desert heat and mountains. 5.13 made it feel 800 lbs lighter and get 17mpg around town and 18plus on trips with steel bumpers, winch, and beadlocks.

Don’t forget to remove the doors for further weight savings

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