Going to 4.88 or 5.13 for 37s?

[kapk22]

Sorry for the stupid question. I’m a little confused. Will going to 4.88 or 5.13s allow the Jeep to see 8th gear more often at higher speeds? Or is it opposite?

I’m on 37s with stock JLUR gears now and see 8th when i am going downhill or between 60-75ish on flat ground and no wind.

Sponsored

 

[Zandcwhite]

You'll see 8th more often as wind resistance from higher speeds creates too much load to cruise at ~1800rpm at 80mph and thus the jeep downshifts. Going all the way to 5.13 makes 8th gear equivalent to your current 7th gear. If you think rpms are too high in 7th at whatever speed you like to cruise at, stick to 4.88s. Everyone gets hung up on seeing 8th gear, but all you are doing on the top end by gearing low is turning 8th gear into 7th. The transmission ratios are so close together between 6th/7th/8th gears that you won't gain any freeway mpg or driveability in my experience and may actually lose some. Low gears help a lot at low speeds and even moderate speeds, but if your only reason for regearing is "to see 8th gear" more, it's a waste in my opinion. All you are doing is guaranteeing your jeep will always be running higher rpms, and sometimes more than necessary as there is no 9th gear.

 

[GRAK]

scroll down

 

[RAMSTEEL]

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

8th gear is .67

Just plug in the other data and you'll see the impact and speeds

 

[kapk22]

Thank you guys.

 

[jhackathorne]

Agreed with others comments. I am in a 2.0L auto at altitude and have no issues running 37's on stock gearing. I don't see 8th gear too often and that is fine with me. I am using a Pulsar XT inline tuner so that helps and I have no plans on regearing either.

 

[azwjowner]

Agreed with others comments. I am in a 2.0L auto at altitude and have no issues running 37's on stock gearing. I don't see 8th gear too often and that is fine with me. I am using a Pulsar XT inline tuner so that helps and I have no plans on regearing either.

The other aspect, often overlooked, is that larger tires are putting increased stress on the output side of the transmission 100% of the time. Whether that leads to premature transmission failure, I have no idea. But even if the gearing in the transmission remains satisfactory, without regearing the axles, the output side is 37/33 = 12% extra torque all the time.

 

[sciotola88]

i love the 5.13 gears we put on our JTR. We ran them with 37 Ko2 and now the 38 Ridge Grappler. It gives you more room should you want to move up a tire size.

 

[Philly_]

General consensus I have seen is people often like 5.13 with the 3.6L and 4.88 with the 2.0L. Sometimes people like to be geared a bit lower or higher and flip flop that, but those are the most common combos.

I don’t think you can go wrong with 4.88s and either engine, but some may feel 5.13 is a bit low.

Full disclosure, I haven’t driven a Jeep with either gear set, just done a lot of research as a regear is planned if/when I go to 37’s.

 

[roaniecowpony]

I just regeared to 4.88s and installed 37s. Frankly, it feels a lot like it did with OEM 33s and 4.10s. Although, numerically, 4.56 with 37s is about the equivalent of 4.10 with 33s. 3.6L/Auto

 

[jhackathorne]

The other aspect, often overlooked, is that larger tires are putting increased stress on the output side of the transmission 100% of the time. Whether that leads to premature transmission failure, I have no idea. But even if the gearing in the transmission remains satisfactory, without regearing the axles, the output side is 37/33 = 12% extra torque all the time.

How much different is this if the weight is the same across sizes I wonder? A 33" BFG KM3 is 68 lb. so only 5 lb. less than my 37's which are the same weight as the 35's I was running. Does diameter play more of a role in this or is it size?

 

[azwjowner]

How much different is this if the weight is the same across sizes I wonder? A 33" BFG KM3 is 68 lb. so only 5 lb. less than my 37's which are the same weight as the 35's I was running. Does diameter play more of a role in this or is it size?

It's mostly diameter because that's the lever that feeds torque into the transmission (all gearing and tires really simplifies to various length levers pushing against each other). A larger diameter tire is like trying to turn a longer lever that has the same weight on the end of it. The inertia of spinning heavy tires likely pales in comparison to the force of the Jeep accelerating. Tire weight is going to wear components that support the weight or push it around like ball joints, tie rods, etc., though.

 

[roaniecowpony]

How much different is this if the weight is the same across sizes I wonder? A 33" BFG KM3 is 68 lb. so only 5 lb. less than my 37's which are the same weight as the 35's I was running. Does diameter play more of a role in this or is it size?

Weight is a much lower order influence on stress for normal rotation of the tire for acceleration and constant speed. It's almost negligible in that regard.

The longer moment arm, a.k.a. the radius increase, is responsible for almost all the extra twisting stress on axles, driveshafts, and transmission/X-fer case shafts. Think of it in reverse of a torque wrench. The longer the wrench, the more torque you apply for the same force at the handle. The handle being equivalent to the tire contact point on the radius of the tire. So, for the same acceleration force at the tire tread, the larger diameter/radius tire will require more torque at the shaft/axle.

 

[snapper692]

My take....

2.0 = 4.88
3.6 = 5.13

 

[CarbonSteel]

There is a statement that is often made here that goes something along the lines of you will simply run in 7th more and not see 8th with an improperly geared Jeep.

I counter that with it is an incorrect assumption to think that you will only drop 1 gear in an improperly geared Jeep (e.g. 4.10/37 versus 4.88 or 5.13/37) with a 3.6L/ZF8 under the same conditions.

In high winds/hills/mountains/altitude a 3.6L/ZF8 Jeep that is geared 4.10/37 will typically be at least 2 transmission gears (or 3) below one that is geared 4.88 or 5.13.

You cannot escape physics...

And my recommendation for 37's is:

2.0T = 4.88
3.6L = 5.13

Sponsored