weight and final gear ratio

[styck.shyfter]

installing new 37s - each wheel/tire is about 110 pounds, also going to be carrying a spare and have a steel front bumper and rock sliders. when does overall weight come into the conversation for gearing? all of local installers i've talked to have said to only go 4.88... when does weight become a factor in deciding on a final gear ratio?

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[Josh00333]

At the beginning of the thought process.

Most shops around here think 5.13 is best for 37" 50% think 4.88 is best for 4.88's.

With the Auto you really can't over gear. IF you run heavy and do mountain driving; Utah, CO etc I would look at 5.13's for 37" personally.

If a MT then 4.88 probably the best "fit" by the charts.

 

[Josh00333]

https://www.jeepgladiatorforum.com/...for-37”-tires-on-17”-wheels-3-6-gasser.81981/

 

[styck.shyfter]

thank you - but all of those charts are based on rpm/mph - i'm trying to understand sprung and unsprung mass (and rolling inertia factors) and where they start to matter when considering re-gearing.

 

[Josh00333]

More mass = more gearing ie torque to get and keep it moving.

We often get to focused on hp, torque is where work is done.

 

[Hootbro]

There is always a trade off when it comes to gears. For actual gear strength and longevity, you want to stay in a lower ratio much as possible. Higher ratios have smaller gear surface engagement as there is only so much real-estate to be able to cut more gears for the higher ratios.

In theory a 4.88 is going to be stronger than a 5.13

 

[Wheelin98TJ]

There is always a trade off when it comes to gears. For actual gear strength and longevity, you want to stay in a lower ratio much as possible. Higher ratios have smaller gear surface engagement as there is only so much real-estate to be able to cut more gears for the higher ratios.

In theory a 4.88 is going to be stronger than a 5.13

I wouldn't run 4.88 over 5.13 for strength.

There are so many running 5.13 without issues. If there is a strength difference, it doesn't seem to matter.

 

[Panthers65]

At the beginning of the thought process.

Most shops around here think 5.13 is best for 37" 50% think 4.88 is best for 4.88's.

With the Auto you really can't over gear. IF you run heavy and do mountain driving; Utah, CO etc I would look at 5.13's for 37" personally.

If a MT then 4.88 probably the best "fit" by the charts.

Agreed- Always overgear on a Jeep. I have small 37's (BFG AT's) and my 5.13s are great.

If you do 90mph everywhere, your mileage is going to suck no matter what. Unless you are 99% interstate with no towing/crawling, go 5.13's at least.

 

[GI Grandpa]

installing new 37s - each wheel/tire is about 110 pounds, also going to be carrying a spare and have a steel front bumper and rock sliders. when does overall weight come into the conversation for gearing? all of local installers i've talked to have said to only go 4.88... when does weight become a factor in deciding on a final gear ratio?

Depends on where you live? Here in Colorado you can get by with 37" tires and 4.88 gears if you're not towing or carrying a lot of extra weight, otherwise 5.13 or even 5.38's would be my choice. One other consideration are your brakes. More unsprung weight means more rolling mass so just as it takes more torque to get moving, it takes more effort to stop. So again, if you're towing or adding a Canopy and a lot of other extras, big brake kit should be on your list too.

 

[CrazyCooter]

You gear to match the diameter of the tire so that rpms put you in the sweet spot. Rolling mass just increases time to reach your target speed.

For auto trucks on 37s, 4.88s for flatlanders and 5.13s for people that tow heavy and/or frequently operate in the mountains.

If you are racing 1/4 mile, obviously thoughts would be different.

 

[styck.shyfter]

so, you're partially correct - rolling mass does increase the duration to get from 0 to Xmph. but the energy required to hold Xmph with a stock weight vs the same jeep loaded with 2000lbs of extra weight is notable - changing the gear ratio doesn't take into account the actual energy required to, say, hold 75mph at different ratios - yes a locked up converter with any given tire size at X rpm, in Y gear with Z final gear ratio will always be the same...

so, although i'm not expecting a complete mathematical answer for every variable - i'm just wondering has anyone on the jt platform, running an automatic, with 37s, regeared and found out that 4.88 was fine until they added whatever equip that started to show degreation in performance. or maybe gone from 4.88 to 5.13s or beyond due to cargo.

also previously mentioned, i already did a big brake kit.

 

[CrazyCooter]

I've had dozens of clients who chose either 4.88's and 5.13's for their JL/JT platforms, but I only push the 5.13's if they are plan to add things that increase wind drag like a roof rack or tow heavy.

4.56's already bring back the final ratio almost perfect to what it was with OEM 4.10's and 33's. 4.88's is that extra 5% bump to help compensate for the little extra weight and wind drag generated by the tires and lift. I have never installed 4.56's on the JL/JT platfom though........not enough jump to justify with the 8 speed handling the shifting IMO.

I find there is a HUGE difference between towing my low profile 6K dump trailer vs my 4K RV. The RV takes much more throttle to keep moving, but the frontal area is probably more than double? Once the dump is rolling, you hardly notice it till the hills come. For the same reason, I chose a bed rack to keep my RTT below the cab roof line.

 

[JT1]

so, you're partially correct - rolling mass does increase the duration to get from 0 to Xmph. but the energy required to hold Xmph with a stock weight vs the same jeep loaded with 2000lbs of extra weight is notable - changing the gear ratio doesn't take into account the actual energy required to, say, hold 75mph at different ratios - yes a locked up converter with any given tire size at X rpm, in Y gear with Z final gear ratio will always be the same...

so, although i'm not expecting a complete mathematical answer for every variable - i'm just wondering has anyone on the jt platform, running an automatic, with 37s, regeared and found out that 4.88 was fine until they added whatever equip that started to show degreation in performance. or maybe gone from 4.88 to 5.13s or beyond due to cargo.

also previously mentioned, i already did a big brake kit.

If you have added 2000 pounds of gear, you are over the weight limit on the chassis.

But, these things are so aero dirty, wind resistance at 80+mph is a bigger issue than weight. Gear it at 5.13 and don't worry about it..

 

[styck.shyfter]

Thank you Crazy for following up on this, these are the thoughts i'm looking for. isn't there about a 5% rpm increase between 4.88 and 5.13? Also thanks for the feedback on your practical experiend. alot of people like to up thier post counts on the forums and are only parroting back what they've read on other threads.

What I don't want to have happen is to throw on my wheels/tires, lift bumper AND THEN toss in another 300-500lbs in passengers and cargo and not be able to HOLD 70 or 75 in 8th without jumping back and forth to 7th. i live in the valley in Arizona but my jeep is a driver, i take it to Colorado and the sierrias multiple times a year. the handfull of shops here say 4.88 and won't acccept any further questions, basically .

 

[CrazyCooter]

I have pretty much checked out of the forums because the lack of actual tech in favor of the parroting. I get disgusted with people discounting or strait up telling me I'm wrong based on what they know from reading all the misinformation. This isn't a "which tire is better" kind of thing.....It's actual factual numbers that can't be disputed.

Here are the numbers rounded:

+9% 3.73-4.10
+10% 4.10-4.56
+6% 4.56-4.88
+5% 4.88-5.13
+4% 5.13-5.38
+16% 4.10-4.88
+20% 4.10-5.13

+11% 33-37" tire

I have a client that built the heaviest 3.6l powered JT I have seen to date, runs the heaviest 37's, roof rack, AND tows a 23' RV....We chose 5.13's for his truck. I actually just questioned him a couple of weeks ago after 4 years of driving it whether he would have chosen 5.38's.....His response is that 5.13's have been perfect and he wouldn't change a thing.

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