Gear ratios and bad advice

[InOmaha]

Sorry got the wrong transmission number. I have a 6 speed. I don't consider 65 highway speeds. I stay in 5th now around town highways until we get to 70mph.

There's no reason to get out of 5th if the rpms are the same. All your doing with a regear is shifting permanently by using a higher transmission gear and lower differential gear. If your 3.07 regear by all means. But going into the .83 overdrive and regearing versus staying in the 1 5th and not regearing can give very similar results overall.

If 5 th gear transmission ratio x differential ratio is the same as 6th gear ratio x differential ratio between two different setups the engine does not know the difference.

If I put 4.88 or 5.13 gears I'd likely never use 1st gear and I'd be topped out in 6th. I'd rather leave the room on the top end and shift down. Then for off roading use my transfer case design to regain the overall powertrain low gear ratio.

If I owned an automatic and daily drove it on the 405 in LA, I'd regear the rear to crawl in traffic at idle.

We were averaging 88mph on our trip last week. So if you guys plan on driving slow through Nebraska on the way to MOAB, do us a favor and take highway 30.

The key to take away though is its not just a gear. It's a system. The system should be looked at as a whole. Each change affects another, and you can change things in different places to get similar results. I rebuilt grim jeepers calculations in a spreadsheet and copied out my specific transmission information. Then looked at my typical driving speeds and shift points. I determined it's a better investment to buy a new transfer case for off reading and use my shifter to compensate for slighter bigger tires.

That gives me the best street and on interstate performance while letting the transfer case handle off road performance.

The affect will be very similar overall performance to 5.13 regearing off road if I left in the 231 but not what I needed on road.

 

[L J]

I'll let everyone look at the charts and base their recommendations off some generic internet information. But it's never been my style not to think it through completely and tailor any final solution to my specific needs.

You go on and think it through completely and we'll continue to recommend the proper gearing based on hundreds of years and thousands of miles of accumulated experience.

 

[InOmaha]

But you couldn't recommend anything until I told you my set up and told you my use. Regearing advice is like someone asking for advice about what truck or gun to buy. You can't tell them until you know more.

You'll pick what works for you based on 100 years of whatever experience works for your specific application. And you'll be right, for your specific application.

 

[Cross-Threaded-06TJ]

I didn't read most of this article, just the first portion where he lists the types of axles for the tires, and it makes no sense to me based on real world. . Currently running 4.10, with 35's, 4.0l, slightly built, and basically stock rebuilt transmission, it runs much better than it did with 4.56. It was almost undriveable on the highway with 4.56 gears. Engine was screaming constantly. I had just come online here to get some info, but it seems there is some tremendous confusion on my part. Having problems with rear end, and thought I should learn some things, but??? I have almost too much low end at every speed now, with 4.10's, and everyone on here seems to think I should have way different ration in direction that was almost undriveable before.

I didn't read most of this article, just the first portion where he lists the types of axles for the tires, and it makes perfect sense to me based on real world. . Currently running 5.13, with 35's, 4.0l, not remotely built, and basically stock never rebuilt transmission, it runs much better than it did with 3.73. It was almost undriveable on the highway with 3.73 gears. Engine was lugging constantly. I had just come online here to get some info, but it seems there isn't some tremendous confusion on my part. Having no problems with rear end, and thought I shouldn't learn some things, but??? I have almost normal low end at every speed now, with 5.13's and everyone on here seems to think I should have way different ration in direction that was almost undriveable before.

Why are you being soo mean, but muh gears! I can downshift. me too.

 

[mrblaine]

But you couldn't recommend anything until I told you my set up and told you my use. Regearing advice is like someone asking for advice about what truck or gun to buy. You can't tell them until you know more.

You'll pick what works for you based on 100 years of whatever experience works for your specific application. And you'll be right, for your specific application.

The problem is always the same. Screaming is mildly revved up to some. Tire size is never accurate. Very few ask what trans and motor combo before they start recommending and rarely does anyone who has good info have experience with the 42RLE.

 

[WallyWest]

There's no reason to get out of 5th if the rpms are the same. All your doing with a regear is shifting permanently by using a higher transmission gear and lower differential gear. If your 3.07 regear by all means. But going into the .83 overdrive and regearing versus staying in the 1 5th and not regearing can give very similar results overall.

Your mistake is focusing on a single speed.

Now if you drive 80+ regularly then fuel economy is obviously not something you care about. But max fuel economy is ALWAYS found in your final gear. The reason to go from fifth to sixth is to save gas. The diff ratio is irrelevant. It is ALWAYS more fuel efficient in sixth vs fifth. Providing of course the speed allows you to be in sixth at all. Which is why regearing frequently increases mpg, by allowing you to get into your final gear sooner.

The full picture isn't found at any one speed though, you have to look at the entire operating range. In your case if you see speeds approaching 90mph frequently and that's the majority of your driving, then ok. But I bet you would still see mpg gains by going to 4.10. Because I would bet your overall average speed is quite a bit less than that.

 

[L J]

But you couldn't recommend anything until I told you my set up and told you my use.

That's why the first things asked when someone posts "what gears" is "which trans" and "what tire size".

You'll pick what works for you based on 100 years of whatever experience works for your specific application. And you'll be right, for your specific application.

It's not me. It's we. It's many varied setups and experiences.

 

[InOmaha]

I'm not opposed to advice at all. But if I was going off of experience alone, then Jeep picked the right ratios for their designs. If I can mimic the overall engine to wheel by changing a few components while increasing off road performance, that seems to be the way to go. I could put on 34" tires, 4.1 gears and sit at 2,800 rpm at interstate speeds here, then shift down at highway speeds. But why regear if the difference isn't much when I can simply not shift.

That's why look at everything through each gear for final driveline ratios, rpms, speeds. Regearing only to shift sooner in traffic doesn't help me. I could regearing for off road or I could put in a different transfercase.

If I changed my 3.73 gears out for 5.13 gears, I'm approaching the same final gear ratio in 2nd gear that I have currently in 1st gear.

So I'd spend all that money to start in 2nd gear instead of first around town, and my crawl ratio for the 231 case in first would go from 50.64 to 69.65. With interstate 80 mph rpm at 3200.

If I instead left in my 3.73 and upgraded my transfer case to a 5:1 atlas my crawl ratio would go to 93.44, and my interstate RPM would be either 2400 in high or 2800 in 5th.

So logic says if I want to offroad spend my money on transfer case and tires first, then when I break crap upgrade axles and regear to something like 4.10. It leaves engine RPM in acceptable ranges for normal driving with a stick. The top gear at higher speeds is the only place I loose flexibility to control rpm as I change the rear differential gearing.

Other people have other considerations. And of course even detailed analysis is generic if you don't doo it all. But I don't feel like pulling out the big books.

 

[Vasq]

isn't it more than just the math the steams from a larger size and gear ratios? I always thought as Jeeps get bigger tires, more lift, more weight, less aerodynamics, more rolling resistance, etc.

doesn't the re-gear help with way more than "the calculator says my rpm's will be at this number with this trans/diff/tire"?

Or I should just stick with lurking for this one

 

[mrblaine]

I'm not opposed to advice at all. But if I was going off of experience alone, then Jeep picked the right ratios for their designs.

Not really. Lots of folks wound up with 3.07 and that doesn't work anywhere even stock except you could downshift and save it.

It is nonsensical to put a combo out there that practically requires you to downshift if a shadow falls across the road and makes it look like it is going uphill slightly.

 

[Cross-Threaded-06TJ]

isn't it more than just the math the steams from a larger size and gear ratios? I always thought as Jeeps get bigger tires, more lift, more weight, less aerodynamics, more rolling resistance, etc.

doesn't the re-gear help with way more than "the calculator says my rpm's will be at this number with this trans/diff/tire"?

Or I should just stick with lurking for this one

You are correct.

The TJ cuts through the wind like a giant brick of cheese in a Hurricane. It needs help. But some like to run 3.07's-3.73's and 33's-35's and not use OD and invent ways to avoid Re-gearing because it's costly and scary voodoo. Also they pretend they go 90MPH on the interstate and its safe.

 

[jjvw]

He's actually right for my application. If I regeared as recommended here I'd be at 3200 rpm at interstate speeds if I drove slow and stayed at the speed limit.

My 32 inch tire, 3.73 gear combination has me at 2684 rpm at 80 mph in 6th gear with the NV3550 manual transmission. If I want higher rpms I can shift down to 5th and be at 3800 rpm.

That's the beauty of a manual. I set the rpm. Not the rear differential gearing. And it's the point being lost. If you drive an automatic regearing is your answer. If you drive a manual, shifting at the same rpm after tire changes is your answer.

Off road if you have 3.73 gears you can buy a 5:1 or 6:1 atlas transfer case to compensate. They cost the same as a 4.3:1. Then on road your jeep drives closer to stock.

But we're mixing advice. Just like the OP breaking down by engine and transmission, we need to add use categories. You can compensate for different on and off road performance issues with different equipment modifications. Axle ratio is only one of several in the powertrain.

With a manual transmission, I can compensate for 30" versus 35" tires by not shifting until I reach the same engine RPMs. Doing so holds the final gear ratio from engine to tire at roughly the same for a given engine RPM and the engine doesn't see too much of a difference. If I regear the rearend for 35" tires I'd likely shift the transmission earlier into a higher gear. Effectively wasting some of my regear.

Automatic transmissions do not give you that sort of flexibility unless you own some of the newer models that allow you to electronically hold the gear.

First gear takeoff with the manual 6 speed and larger tires might be a point I can't control, but 1st gear is already low so up to 35" tires shouldn't be noticeable. I might not have to shift before I get through intersections anymore. But regearing has similar affects on the high end side when you get to that limit.

When we lay out our gear recommendations by transmission, it's helpful to add the gear ratios of each transmission gear. It's why grim jeepers site is so helpful. I can look at my individual traffic patterns and find speed rpms for each gear point. I'd hate to select a rear differential gear that had me shifting up and down between 2nd and 3, or 3 and 4, while stuck in traffic during in my normal daily drive. Who needs that sort of constant clutching.

I'll let everyone look at the charts and base their recommendations off some generic internet information. But it's never been my style not to think it through completely and tailor any final solution to my specific needs.

If I leave my 3.73 gears in and put on 35" (likely 34") tires my engine RPM at interstate speeds would be 2450 RPM and I could stay down a gear and rev up to 2950 RPM if needed. Not significantly different than my current setup which works for me all around. Then when I go transfer case shopping I'll get a 5:1 ratio instead to compensate for my rear differential gearing. That would come close to doubling my current crawl ratios.

If you have the nv3550, you don't have 6th gear.

 

[jjvw]

I'm not opposed to advice at all. But if I was going off of experience alone, then Jeep picked the right ratios for their designs. ....

Which of these is the correct design? How is it that all Rubicons came from the factory with one size tire, one axle gear ratio and three different transmissions and all three are the right ratio for it's design?

42rle / 29.5" /3.73 = 2052rpm @ 70mph
NV3550/29.5"/3.73 = 2320rpm @ 70mph (nearly 300rpm higher than the 42rle)
AX-15 / 29.5" /3.73 = 2349rpm @ 70mph (nearly 300rpm higher than the 42rle)
NSG370/29.5"/3.73 = 2498rpm @ 70mph (over 400rpm higher than the 42rle)
32rh / 29.5" / 3.73 = 2974rpm @ 70mph (nearly 900rpms higher than the 42rle)

 

[Cross-Threaded-06TJ]

Which of these is the correct design? How is it that all Rubicons came from the factory with one size tire, one axle gear ratio and three different transmissions and all three are the right ratio for it's design?

42rle / 29.5" /3.73 = 2052rpm @ 70mph
NV3550/29.5"/3.73 = 2320rpm @ 70mph (nearly 300rpm higher than the 42rle)
AX-15 / 29.5" /3.73 = 2349rpm @ 70mph (nearly 300rpm higher than the 42rle)
NSG370/29.5"/3.73 = 2498rpm @ 70mph (over 400rpm higher than the 42rle)
32rh / 29.5" / 3.73 = 2974rpm @ 70mph (nearly 900rpms higher than the 42rle)

The AX-15.

 

[Vasq]

i thought Rubicon's came with 4.10

 

[InOmaha]

Seems to work okay now. But I guess the thread isn't about technical info. Just cheese.

The torque curve of the 4.0 is rather flat. But it does peak out at around 3000 rpm. I just think it's incredible in this discussion that people can't figure out that regearing the differential is to reset the overall gear ratio of the power train to get RPMs in the same 2400-3200 peak torque range as the original design and that they don't understand that you can get the same (or similar) overall powertrain ratios with different transmission gears. If the overall ratio is the same, the engine doesn't know the difference from clutch plate to wheel. So bigger tires drops rpms and your regearing to hit the sweet spot. But a stick stick allows me to regear on the fly to get my rpms in close enough to the same range.

If you regear enough to the point you're basically up one gear level you will do nothing for your daily driving except start in a higher gear and run yourself onto the downward slope of the torque curve at higher speeds. Overall engine efficiency of ICE engines is almost always ~75% load. So if you could find that curve somewhere, but I can almost guarantee you it's somewhere around 2,800-3000 rpm range the torque drops at 3,000. Somewhere around 140hp on these piggy engines probably.

I've tried to simply it some not to bore people but I'm going intake to tire contact. I'm setting engine RPM using tire size and gearing options for highway, setting off road by selectable gearing in the transfer case, and using the manual transmission as a buffer between upgrades.

I'm not looking at fuel economy other than to not purposefully make it too much worse.

I think 2500 rpm at speed in 6th is a little low but not worth the money now. I can shift to 5th and bump to 2900 rpm when needed to get me on the right spot for the engine. That's on 35" tires coming in below 35" effective radius. Regearing is doing the exact same thing as shifting down into a lower gear on a manual transmission. Instead of regearing the axles you could regear or replace the transmission and get the same results.

I don't have a problem now on 32.1" (33") compared to stock tires because I'm do the same thing. I'll simply relearn my shift points.

 

[jjvw]

i thought Rubicon's came with 4.10

Yes. I can do the math if you need me to.

 

[jjvw]

Seems to work okay now. But I guess the thread isn't about technical info. Just cheese.

The torque curve of the 4.0 is rather flat. But it does peak out at around 3000 rpm. I just think it's incredible in this discussion that people can't figure out that regearing the differential is to reset the overall gear ratio of the power train to get RPMs in the same 2400-3200 peak torque range as the original design and that they don't understand that you can get the same (or similar) overall powertrain ratios with different transmission gears. If the overall ratio is the same, the engine doesn't know the difference from clutch plate to wheel. So bigger tires drops rpms and your regearing to hit the sweet spot. But a stick stick allows me to regear on the fly to get my rpms in close enough to the same range.

If you regear enough to the point you're basically up one gear level you will do nothing for your daily driving except start in a higher gear and run yourself onto the downward slope of the torque curve at higher speeds. Overall engine efficiency of ICE engines is almost always ~75% load. So if you could find that curve somewhere, but I can almost guarantee you it's somewhere around 2,800-3000 rpm range the torque drops at 3,000. Somewhere around 140hp on these piggy engines probably.

I've tried to simply it some not to bore people but I'm going intake to tire contact. I'm setting engine RPM using tire size and gearing options for highway, setting off road by selectable gearing in the transfer case, and using the manual transmission as a buffer between upgrades.

I'm not looking at fuel economy other than to not purposefully make it too much worse.

I think 2500 rpm at speed in 6th is a little low but not worth the money now. I can shift to 5th and bump to 2900 rpm when needed to get me on the right spot for the engine. That's on 35" tires coming in below 35" effective radius. Regearing is doing the exact same thing as shifting down into a lower gear on a manual transmission. Instead of regearing the axles you could regear or replace the transmission and get the same results.

I don't have a problem now on 32.1" (33") compared to stock tires because I'm do the same thing. I'll simply relearn my shift points.

All I know is that with my 5 speed manual I liked 4.88/33s better than 4.88/35s. This statement includes flat highway driving, mountain highway driving and city driving. Based on my experience, I have every intention of regearing to at least 5.13. And 5.38 is not out of the question. And fwiw, my Jeep drives very well at 75mph and 80 on occasion. There is nothing in my experiences that suggests less gear would be better.

 

[never monday]

<— 42RLE, 3.73, 31’s @ 2000 to 5000ft above sea level. It sucks.....

Omaha elevation 1,089’

 

[Cross-Threaded-06TJ]

Seems to work okay now. But I guess the thread isn't about technical info. Just cheese.

The torque curve of the 4.0 is rather flat. But it does peak out at around 3000 rpm. I just think it's incredible in this discussion that people can't figure out that regearing the differential is to reset the overall gear ratio of the power train to get RPMs in the same 2400-3200 peak torque range as the original design and that they don't understand that you can get the same (or similar) overall powertrain ratios with different transmission gears. If the overall ratio is the same, the engine doesn't know the difference from clutch plate to wheel. So bigger tires drops rpms and your regearing to hit the sweet spot. But a stick stick allows me to regear on the fly to get my rpms in close enough to the same range.

If you regear enough to the point you're basically up one gear level you will do nothing for your daily driving except start in a higher gear and run yourself onto the downward slope of the torque curve at higher speeds. Overall engine efficiency of ICE engines is almost always ~75% load. So if you could find that curve somewhere, but I can almost guarantee you it's somewhere around 2,800-3000 rpm range the torque drops at 3,000. Somewhere around 140hp on these piggy engines probably.

I've tried to simply it some not to bore people but I'm going intake to tire contact. I'm setting engine RPM using tire size and gearing options for highway, setting off road by selectable gearing in the transfer case, and using the manual transmission as a buffer between upgrades.

I'm not looking at fuel economy other than to not purposefully make it too much worse.

I think 2500 rpm at speed in 6th is a little low but not worth the money now. I can shift to 5th and bump to 2900 rpm when needed to get me on the right spot for the engine. That's on 35" tires coming in below 35" effective radius. Regearing is doing the exact same thing as shifting down into a lower gear on a manual transmission. Instead of regearing the axles you could regear or replace the transmission and get the same results.

I don't have a problem now on 32.1" (33") compared to stock tires because I'm do the same thing. I'll simply relearn my shift points.

It is about cheese generally at this point.