37s and questions

[3TV]

In my 2-dr JLR with a 2.0T, 8 speed auto, and 4.10 gears I switched from the stock 33" tires to 37" tires and noticed a definite decrease in power that I did not like. I regeared to 4.88 gears, and it felt good again. So, then I changed to 39" tires . It is still good enough that I am not going to regear again. It easily hits 8th gear at 80 mph on the interstate. And with the 4.0:1 T-case it is good offroad as well.

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[tk30184]

Been watching all the posts with little or no time to reply until now. I’m new to jeepin’ and forums so I missed providing some info. I assumed if anyone cared that my profile would give the basics but I was wrong.
Anyway… live in north Georgia so it’s basically sea level. It’s a 19 JLUR. 8 spd AT, 4” lift with 37x13.5x20 Toyo Open Country RTs. Moto Metal wheels.
I bought this rig as-is. I love it. It feels like it runs great but I also think it could run better. I’ve only owned it a few months.
I drove the same truck with a 4 speed AT for 25 years so this is new getting used to all the shifting. (I ran 35s with 4.10 gears in my truck)
My work commute is 3 miles round trip and since I bought it I’ve only had a few chances to wind it out. I do get to see 7th and 8th now and again but not often and not for long.
What I’m getting is that you can be super technical, or go with what you got if you like it. What I can say for sure is that the best mpg I’ve seen so far is 13.7. That’s after driving to Atlanta but since the traffic always sucks there it wasn’t a consistent highway speed.
The reply I like best is that going back to 35s would be like putting a smaller TV in your living room.
That said, I love the way it looks so I’m gonna try going to a 4.88 in the future. I’m not made of money so it might take a while. I’ll research tires more between now and then also. I’ve never had Toyos before. I inherited these so I’ll see how it goes.
I plan to keep it a pavement princess for a while until it gets up over 120k miles. (Currently 70k) Maybe I’ll get another princess and then start modding this one out to hit some technical trails. Money and time will dictate all that. Since I’ve been waiting 25 years to join the Jeep club I’ve learned to be patient.

 

[3TV]

Since your Jeep has a 4" lift, going back to 35s will make it look like it skipped leg day at the gym. I say run it as is, or gear down to 4.88s, but don't downsize your tires. Toyo Open Country RT tires are great tires by the way.

 

[AmericanPatriot100]

FWIW my 25 2dr sport 6MT on the stock 245/75/17 hwy tires and 3.45 gears never saw 5th cruising on the hwy even going long distances. And then I switched to the MTBBAT in an LT255/85/17 and I actually use 5th gear more often so the mpg stayed them same. Might have gotten better by .2 or.3 mpg in town and 2mpg on long trips. I know that sounds backwards but it’s what I’ve experienced. And I’ve had it this way for 15k miles. Consistently see between 16.7 and 17.7mpg depending on traffic. Which since I haven’t fixed my Speedo I’m sure the mpg is higher than what I’m actually getting. That said I do plan to regear.

 

[wibornz]

So, I have run 37s with 4.10 gears and 5.13 gears. Is the Jeep faster with 5.13 gears, yes. Is it a big deal, no. I very rarely do full pulls to 60 mph.

Zero to 60 time with 4.10 gears

Zero to sixty time with 5.13 gears


The 5.13 gears shaves off almost a full second in zero to sixty times. Who cares. If you bought a Jeep to hot rod, you are making poor decisions anyways as the Jeep with 5.13 gears is still pathetically slow.

I have wheeled Moab with 4.10 gears and 5.13 gears. Sure it crawls better with 5.13 gears. Is it a game changer, no. The 3.6 motor will still spin all four tires when it is locker on slick rock with ease even with 37s aired down to 8 psi. The 8 speed transmission is amazing and the 4 to 1 transfer case is great. It will make up for the 4.10 gears in all but the most challenging situation.

I found pulling a camper over the mountain passes in Colorado to be a challenge with the 4.10 gears. So I jumped to 5.13 gears. What I found is that with 4.10 gears, I would be running about 45 mph up the pass in third gear about 4,000 rpm. With 5.13 gears, I would be running in about 45 mph in 4th gear......at about 4,000 rpm. Regrearing the JLUR on 37s, I found that it just changed what gear the transmission use the most often as it was the 3.6 hp output that was the limiting factor. Like 7th gear was more like 8th gear after the regear. 6th was more like 5th and so on and so. In driving, I only really noticed the difference in everyday driving in the first through third gear. The Jeep was faster off the line and in speeds under 45 mph. After 45 mph, the transmission was flexible enough in gear selection that it did not matter that much in daily driving.

 

[tk30184]

So, I have run 37s with 4.10 gears and 5.13 gears. Is the Jeep faster with 5.13 gears, yes. Is it a big deal, no. I very rarely do full pulls to 60 mph.

Zero to 60 time with 4.10 gears

Zero to sixty time with 5.13 gears


The 5.13 gears shaves off almost a full second in zero to sixty times. Who cares. If you bought a Jeep to hot rod, you are making poor decisions anyways as the Jeep with 5.13 gears is still pathetically slow.

I have wheeled Moab with 4.10 gears and 5.13 gears. Sure it crawls better with 5.13 gears. Is it a game changer, no. The 3.6 motor will still spin all four tires when it is locker on slick rock with ease even with 37s aired down to 8 psi. The 8 speed transmission is amazing and the 4 to 1 transfer case is great. It will make up for the 4.10 gears in all but the most challenging situation.

I found pulling a camper over the mountain passes in Colorado to be a challenge with the 4.10 gears. So I jumped to 5.13 gears. What I found is that with 4.10 gears, I would be running about 45 mph up the pass in third gear about 4,000 rpm. With 5.13 gears, I would be running in about 45 mph in 4th gear......at about 4,000 rpm. Regrearing the JLUR on 37s, I found that it just changed what gear the transmission use the most often as it was the 3.6 hp output that was the limiting factor. Like 7th gear was more like 8th gear after the regear. 6th was more like 5th and so on and so. In driving, I only really noticed the difference in everyday driving in the first through third gear. The Jeep was faster off the line and in speeds under 45 mph. After 45 mph, the transmission was flexible enough in gear selection that it did not matter that much in daily driving.

Now that’s some good detailed feedback, and honestly kind of what I was expecting. I think for my wants and needs the 4.88 will do the trick. Sounds like there’s always a compromise and something will always get left on the table. If you do a little of EVERYTHING then you’ll find where it’s strong points and weak points are or were. Thanks for all the info.

 

[tk30184]

NEVER skip leg day!

 

[Bdoza10]

Just a heads up, you can't change shift points with JScan, you can only program the computer to correct the speedo. Which will give a sensation that your shift points are better, but it just seems that way because your speedo is correct, the TCM is still commanding the shift at the same output shaft speed (which is your driveshaft speed). On this platform with the 8 speed, there is no correlation between your speedo being accurate and the transmission deciding when to shift either, this is a false statement made by multiple people on this and other forums. Shift RPM differenes are simply because of the regearing and not something that will change with settings in Jscan.

I've tuned a bunch of these now ranging from 31.5s on 3.45s to 40s on 5.38s. For 37s, 4.88s seem to make everyone the happiest, but 4.10s are tolerable with good engine and transmission tuning. I've got two customers running 40s paired with 5.38s and they love it.

Thoughts on the 2.0 with 4.56’s and the light k02 37’s and some 21lbs rims?

 

[bmpcamry09]

Thoughts on the 2.0 with 4.56’s and the light k02 37’s and some 21lbs rims?

You’d probably be fine. I’d normally recommend 4.88s with 37s but with the lighter gears and the lower torque curve of the 2.0 it’d probably be manageable. I still highly recommend a transmission tune though

 

[kah.mun.rah]

Thoughts on the 2.0 with 4.56’s and the light k02 37’s and some 21lbs rims?

If you are driving a Sport you will notice a big difference if you re-gear with 37s.

 

[Bdoza10]

If you are driving a Sport you will notice a big difference if you re-gear with 37s.

I have xr 392 axles 4.56 gears. But yea other than that it’s a very light set up. I’m just torn between going Goodyear 315/70/17 55lbs or Bfg ko2 37 12 17 64lbs. Grim jeeper has me leaning 37’s. I plan on running scs rims that are only 21lbs. And oddly enough I have spare ti lugs from another project that are about half the weight of the stock ones.

 

[kah.mun.rah]

I have xr 392 axles 4.56 gears. But yea other than that it’s a very light set up. I’m just torn between going Goodyear 315/70/17 55lbs or Bfg ko2 37 12 17 64lbs. Grim jeeper has me leaning 37’s. I plan on running scs rims that are only 21lbs. And oddly enough I have spare ti lugs from another project that are about half the weight of the stock ones.

Go with the 37s and with 4.56 you will be fine (especially if you drive a lot of flat sea-level miles).

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