37 vs 38’s and 4.88 vs 5.13

[gearhead22]

Here’s my dilemma, with the 8 speed and 4.10’s on 37’s my rpm’s at 70 in 7th gear are basically what they would be in 8th gear with 5.13’s. With 5.13’s I’m maxed out at that point in terms of transmission gears, while with the 4.10’s I still have the ability to get into 8th when all stars align (down hills and such). Would regearing really help much? Keeping it as is basically I lose 8th gear the majority of the time, but in 7th the rpm’s would be the same and the trans ratio is slightly less overdriven which would result in more power to the tires if I’m thinking correctly? I don’t want to cause any harm to the jeep by not regearing. My main commute is 5 miles to work with an occasional 20-30 minute highway trip a couple times a week. We do road trip a few hours away every so often so I want it just as good on road as off. Regearing is about 27-3000 in my area so it’s not a cheap venture.

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[Panthers65]

Its the same dilemma as every other lifted Jeep on the road. Not being mean or anything, but you aren't special. 99% of the time no one ever regrets a regear, it's the cheapest bang for your buck to put some umph back in the seat while driving, it helps move the heaver weight of larger tires/winch/RTT/whatever heave accessories you bolt to your Jeep, it saves transmissions, there's just really no downside to it other than the cost.

I'm running 5.13's and a small 37. I run 70-80mph for 8 miles every morning to work, and another 7-8 miles on streets. You're no where near "maxed out" with 5.13's unless your running 110mph everywhere you go, in which case you REALLY bought the wrong vehicle. Don't be afraid of these engines revving up a little, they make the most efficient power around 2400-2500rpm

 

[gearhead22]

Its the same dilemma as every other lifted Jeep on the road. Not being mean or anything, but you aren't special. 99% of the time no one ever regrets a regear, it's the cheapest bang for your buck to put some umph back in the seat while driving, it helps move the heaver weight of larger tires/winch/RTT/whatever heave accessories you bolt to your Jeep, it saves transmissions, there's just really no downside to it other than the cost.

I'm running 5.13's and a small 37. I run 70-80mph for 8 miles every morning to work, and another 7-8 miles on streets. You're no where near "maxed out" with 5.13's unless your running 110mph everywhere you go, in which case you REALLY bought the wrong vehicle. Don't be afraid of these engines revving up a little, they make the most efficient power around 2400-2500rpm

Peak torque is 4400 so no matter what you’re nowhere near that. By maxed out I mean there’s no more gear. It’s basically the same as being in 7th with 4.10’s, except you’d have an extra gear. With 4.10’s you basically lose 8th. But with 5.13’s you basically lose 1st UNLESS you’re offroad and rock crawling a lot.

 

[Panthers65]

Peak torque is 4400 so no matter what you’re nowhere near that. By maxed out I mean there’s no more gear. It’s basically the same as being in 7th with 4.10’s, except you’d have an extra gear. With 4.10’s you basically lose 8th. But with 5.13’s you basically lose 1st UNLESS you’re offroad and rock crawling a lot.

I didn't say peak torque, you don't need to run peak on the interstate. I said most efficient. The torque curve rises and flattens out around 2500 RPM, so while you might make a bit more at 4400, it's relatively the same as it is at 2500, so the gains are minimal past 2500 compared to the extra RPMs.

Regearing helps you see 8th gear, but what it really does is regains lower gears you lost with the larger tires. 37's on 3.73's is like starting a factory Jeep in 2nd or 3rd gear, or having factory 2.73's on factory tires. Regearing helps much more with low it, getting things moving, and minimizing heat in slow speed higher torque situations. It also addresses the fact that your moving substantially more weight than you were on the factory tires/wheels.

You absolutely don't "lose first". If you overgear, your first might turn more into a "granny gear" of old, but your first is stronger, not "lost"

 

[gearhead22]

I didn't say peak torque, you don't need to run peak on the interstate. I said most efficient. The torque curve rises and flattens out around 2500 RPM, so while you might make a bit more at 4400, it's relatively the same as it is at 2500, so the gains are minimal past 2500 compared to the extra RPMs.

Regearing helps you see 8th gear, but what it really does is regains lower gears you lost with the larger tires. 37's on 3.73's is like starting a factory Jeep in 2nd or 3rd gear, or having factory 2.73's on factory tires. Regearing helps much more with low it, getting things moving, and minimizing heat in slow speed higher torque situations. It also addresses the fact that your moving substantially more weight than you were on the factory tires/wheels.

You absolutely don't "lose first". If you overgear, your first might turn more into a "granny gear" of old, but your first is stronger, not "lost"

As stated, lost is relative. It will make it as useless as 8th with 4.10’s and 37’s unless in certain conditions ie rock crawling or very low speed crawling. You may use it from a dead stop under normal driving but you will be into 2nd gear much quicker.

 

[Zachanadandy]

As stated, lost is relative. It will make it as useless as 8th with 4.10’s and 37’s unless in certain conditions ie rock crawling or very low speed crawling. You may use it from a dead stop under normal driving but you will be into 2nd gear much quicker.

The extra umph of regearing is a myth. I measured 0-60 via the tazer before and after the regear and gained 1/10 of a second. There's a guy in the JL forum that did the same via the dragy (more accurate and with a graph of each run from 0mph). He saw the same results and the only advantage is the slight gain at initial takeoff. Logic would tell you that via the fact that the 8 speed has so many gears. 1st gear is the only place you gain anything that a simple downshift doesn't gain you with taller gears. Bang for the buck? I'd call regearing the worst bang for the buck mod I've ever done on a modern vehicle with an 8 speed. You'd gain more umph and 0-60 with a tune from Livernoise for under $1k. The quicker shifts, especially from a stop, are what gives people the illusion of a power gain. It's running through the gears faster so it must be quicker right? Or by simple math is just making each shift at a lower speed. You won't hurt anything by not regearing. The Jeep does a fantastic job of keeping the engine in the power band when needed. Who cares if it downshifts to 4th pulling a really steep grade at speed where the regeared truck might only downshift to 5th? The rpms and load on the engine will be the same. The engine doesn't know if the transmission is in 6th gear with 3.45 axle gears or in 8th gear with 5.13s. It sees the same final drive ratio of 3.45. And with our 4.7-1 1st gear even from a stop you have the same gearing as a TJ or early JK auto with 6.74 axle gears. Take a look at any JK or TJ toy see on the trail on 37-38s. Their probably ruining the same 5.13s that some JT guys think you need with the 8 speed because math is hard. You can add legitimate power via a tune for 1/3 the cost or 50% more power for twice the cost of a regear via a supercharger. Even if regearing was free I'd only go to 4.88s for anything smaller than 40s.

 

[Deleted member 67086]

 

[Badunit]

One thing I paid attention to when choosing gears was staying below the rpm where the cam is in high lift mode. I believe it is 2700rpm. If you give it a margin of error and go with 2500 rpm as cruising rpm then top speed for an automatic will be:

4.88 with 37's = 80mph (2700 rpm = 85mph)
5.13 with 37's = 85mph (2700 rpm = 90mph)

I have an automatic with 4.56 and the stock 33's, which is the same overall gearing as 5.13 with 37's. I like it a lot better than the 4.10 it came with but it eliminated cruising at 85mph (which is 15 over the speed limit here anyway).

Top gear for automatic is 0.67. Top gear for manual is 0.72. A lot of times people don't mention which they have.

 

[Deleted member 67086]

Here’s my dilemma, with the 8 speed and 4.10’s on 37’s my rpm’s at 70 in 7th gear are basically what they would be in 8th gear with 5.13’s. With 5.13’s I’m maxed out at that point in terms of transmission gears, while with the 4.10’s I still have the ability to get into 8th when all stars align (down hills and such). Would regearing really help much? Keeping it as is basically I lose 8th gear the majority of the time, but in 7th the rpm’s would be the same and the trans ratio is slightly less overdriven which would result in more power to the tires if I’m thinking correctly? I don’t want to cause any harm to the jeep by not regearing. My main commute is 5 miles to work with an occasional 20-30 minute highway trip a couple times a week. We do road trip a few hours away every so often so I want it just as good on road as off. Regearing is about 27-3000 in my area so it’s not a cheap venture.

Mine had 3.73s from the factory and I upgraded to 35” tires and a 2.5” lift. I drove it that way a couple months. It was not terrible but I definitely noticed it was more sluggish taking off from a stop, and of course 8th and a lot of times 7th gears were nonexistent on the highway, which I drive a lot.

When I started contemplating a regear, a lot of people recommended 4.88s, but like you, I rationalized that if I go 4.88s, I would be pulling about the same RPM on the highway in 8th gear, as I was in 6th and 7th gear with my stock gears. Having looked at a number of formulas that recommend gears based on current gears and new tire sizes, 4.56 gears made more sense for my situation, which is what I went with. It kept my RPM at highway speeds almost identical to what they were stock, and I am more than happy with the decision. That said, as I mentioned in a previous post there are a lot of variables when determining gearing. Tire weight, vehicle weight, engine, transmission, vehicle use (ex. towing), all factor in.

I understand your apprehension, that price range you mentioned is pretty steep. I paid roughly $2,100 last year for mine, so a bit of an easier decision.

For me the pros of regearing was the Jeep basically ran/drove like it did from the factory. Do you have to regear? No, you will find no shortage of people who’ve not regeared with bigger tires. For me it made sense that engineers determine a vehicles gearing based on a number of variables, tire sizes being one of them. Changing the tire size throws that equation out of wack.

If you do decide to regear, I found the following tool useful for looking at current numbers and comparing them with different gears. Good luck!

https://www.grimmjeeper.com/gears.html

 

[gearhead22]

Mine had 3.73s from the factory and I upgraded to 35” tires and a 2.5” lift. I drove it that way a couple months. It was not terrible but I definitely noticed it was more sluggish taking off from a stop, and of course 8th and a lot of times 7th gears were nonexistent on the highway, which I drive a lot.

When I started contemplating a regear, a lot of people recommended 4.88s, but like you, I rationalized that if I go 4.88s, I would be pulling about the same RPM on the highway in 8th gear, as I was in 6th and 7th gear with my stock gears. Having looked at a number of formulas that recommend gears based on current gears and new tire sizes, 4.56 gears made more sense for my situation, which is what I went with. It kept my RPM at highway speeds almost identical to what they were stock, and I am more than happy with the decision. That said, as I mentioned in a previous post there are a lot of variables when determining gearing. Tire weight, vehicle weight, engine, transmission, vehicle use (ex. towing), all factor in.

I understand your apprehension, that price range you mentioned is pretty steep. I paid roughly $2,100 last year for mine, so a bit of an easier decision.

For me the pros of regearing was the Jeep basically ran/drove like it did from the factory. Do you have to regear? No, you will find no shortage of people who’ve not regeared with bigger tires. For me it made sense that engineers determine a vehicles gearing based on a number of variables, tire sizes being one of them. Changing the tire size throws that equation out of wack.

If you do decide to regear, I found the following tool useful for looking at current numbers and comparing them with different gears. Good luck!

https://www.grimmjeeper.com/gears.html

Thanks! Yeah there’s a very limited amount of shops around me that have the knowledge to do it. The going rate is about 800 per axle plus parts.

 

[Panthers65]

......

I understand your position, and I think you and I have gone around on this topic before. Fact is 75 years of professional racers, jeep builders, and drag car racers don't lie. "Economic" cars come with high ratio gears for the mileage at the expensive of performance. Deeper gears have been put in performance vehicles with a higher price tag than I'd bet anyone on this site will ever be able to afford. We can talk about the subtle differences between the gladiators and all the vehicles that came before it, but to say swapping from a set of 4.10s to 5.13's had negligible change on a vehicle aside for raising the RPMs is ludicrous and simply defies the laws of physics

 

[Zachanadandy]

I understand your position, and I think you and I have gone around on this topic before. Fact is 75 years of professional racers, jeep builders, and drag car racers don't lie. "Economic" cars come with high ratio gears for the mileage at the expensive of performance. Deeper gears have been put in performance vehicles with a higher price tag than I'd bet anyone on this site will ever be able to afford. We can talk about the subtle differences between the gladiators and all the vehicles that came before it, but to say swapping from a set of 4.10s to 5.13's had negligible change on a vehicle aside for raising the RPMs is ludicrous and simply defies the laws of physics

Numbers don't lie. I guess the tazer and dragy both malfunctioned the same for 2 different people who don't know each other? Both people who spent the money on regearing only to be underwhelmed. Who cares about 75 years of history with 3 and 4 speed autos, it's irrelevant. The math didn't lie. The autos in tjs, xjs, and early.JKs all had a 2.86:1 1st gear. Even geared all the way to 5.38s they had the same 1st gear final drive ratio as a JL or JT with 3.27 axle gears. And yet you'll see all those vehicles crawling all over the world? It's not defying the laws of physics but adhering to them. Final drive ratio is all the engine sees. You already have plenty of gear selection available for any final drive ratio you could even need. Outside of 1st gear there's nothing a regear dies that a downshift doesn't. It's simple math. And if you go off the deep end you'll only gain 1mpg around town and lose 2-3 on the highway. These are facts from my own experience going from 4.10s to 5.38s on 38s. There's no point of having 8 speeds when it's run through all of them by 40mph. Half the speed limit isn't where I want to be in my 2nd overdrive gear in an 8 speed which is why they don't give us 4.88s with 33s. Nothing to be gained.

 

[Panthers65]

...

ok

 

[KidBobot]

Wow... I went with 4.56 on 37's and no complaints. I'm 95% on relatively flat ground here in NYC.

 

[Hemy5587]

Stock 3.73s and C rated 37" BFG K02s gasser... No regrets. I get a legit hand calculated 18.1-18.4 mpg week after week, fill up after fill up (until winter gas comes back). My daily commute is 50/50 city/hwy around 40 miles round trip. I've never pulled out in traffic and wished for more and never thought I was underpowered pulling my tractor on my trailer or boat. I am the minority though. I call bs on the guy getting 18 mpg with 5.13s & 37s unless you don't go over 55 mph, for highway speeds your RPMs are too high for that kind of mpg or you are a diesel. Just not possibly mathematically, deeper gearing + faster speeds means more RPM = more gas burned plain and simple.

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