Gear ratios and bad advice

Brantley

TJ Enthusiast

Please know what you're talking about before making a video to the entire world about something. While the intentions of this video are good, and Bleepin' Jeep usually has good information for its viewers, I don't think I've ever groaned more over a Jeep topic than I did watching this video.

This video is full of misinformation. The worst part about it is the multitude of comments from people who had no idea what gear to run and are now thanking him for "helping" them pick out what they should run. Those will be some sorely disappointed folks when they realize how bad his advice was.

He starts by teaching one how they "should" calculate their new gear ratio. He bases his calculations off of stock gears (3.07 in several cases) and uses 29" tires as the basis for his calculations. This is problematic for several reasons.

1) 3.07 was a horrible gear ratio, even for stock tires. On 29" tires with the AX15, you end up right at 2200 RPM at around 70 in 5th gear which lugs the engine. Not good.

He ends up doing the math for 33's which results in him giving 3.55 as a recommendation for 33" tires. I don't know if any of you have tried that ratio, but it sucks.

So, he's already basing his recommendation off of a pretty poor example. If you want to do the math equation, start with Rubicon gearing. The 5-speed manual Rubicon TJs were really the only Jeeps properly geared from the factory in my opinion. The automatic should have had 4.56 for 31" tires, not 4.10 like its manual counterpart.

2) He adds nothing to compensate for larger, heavier tires, extra weight on the Jeep, lift height pushing it into the wind stream, etc. The lift height is almost negligible but the other items I mentioned make a huge difference.

3) His 2.5 example is flawed for several reasons.

First, he starts with using his YJ as an example. He uses 29" tires and 4.10 gears to describe why his 4.88 on 33" tires has the RPM "through the roof".

The flaws begin with the tire size. The largest tire size on a 2.5 YJ was 205/75R15 (27.1"), not 29". There was an option for 215/75R15 (27.7") which my 4.0 YJ had but I believe that was for the 4.0 only. So, he's already nearly 2" off on his calculation method. That makes a huge difference in what he's going to recommend.

Next, he does his math using 29" tires and 4.10. He ends up getting a number of 4.95 for 35" tires. Rather than going to the lower option of 5.13, he chooses to drop back to 4.88. Bad decision, that means if his Jeep came stock with 4.10 and 29" tires, he would now have even less gearing with 35's than he did stock on those 29" tires that don't exist on the YJ model.

He follows this up by saying he regeared to 4.88 expecting to run 35's but ended up with 33's instead because of a deal he got or something. He proceeds by saying his RPM is way too high and that he hits 3000 RPM at 55 mph. False, that setup doesn't reach 3000 until about 69-70 mph.

He also says his poor gas mileage comes from running 3000 RPM, also false, the 2.5 did more than that stock and it sure didn't ever hurt gas mileage. That's the nature of the beast on a 2.5.

4) Real world vs. math. Numbers are good but gear ratios on paper are nowhere close to realistic when you actually drive the Jeep with all the variables the world throws at it.

You can't tell me that 3.55 gear ratio he recommended for his 33's will end up driving well or even close to it.


To end this long rant, don't listen to the people telling you to use your stock gear ratio to determine a new gear ratio for a larger tire. It may add up on paper but it doesn't translate properly in the real world.

The only good combination from the factory was the 5-speed Rubicon and then some of the 32RH combinations.

One other combo that was okay when stock was the 5-speed 3.55 package that came with about 27.7-28" tires. However, you can't follow this for larger ties because it'll end up at 4.10 for 33's, 4.56 for 35's, etc., which aren't ideal setups.

There should have never even been 3.07 offered at all in my opinion.

I sure hope that YouTube guy has played around with his ratios and tires a few more times since the production of this video and has figured out that his advice was very misleading not ideal in the slightest.

Things like this where the people giving out info seem very knowledgeable end up convincing people who don't know an better that they can listen to the person who speaks in confidence, even if they aren't right. Since this guy had some confidence in the things he said, I wouldn't be surprised at all if he lead quite a few people down the wrong road of picking the proper gear ratio if they chose to regear their axles.

Bottom line, listen to people who have real world experience with all the different ratios and tire sizes, not the guy on YouTube using math to figure out what he'll run. There are quite a few very experienced guys out there who can steer you to the exact ratio you want off of the top of their head. Those are the guys you want to listen to.

Rant over.

Just to provide something useful out of this post, here are what I and many other forum members have determined to be the best ratios for Wranglers older than 2007 that have the 4.0L, 2.5L, or 2.4L engines.

4.0L engine

5-speed manual (AX15/NV3550)

31's - 4.10 (this is how the Rubicon came)
33's - 4.56
35's - 4.88
37's - 5.13 or 5.38 (personally I'd choose 5.38 which adds about 100 RPM to help for the larger 37" tires)

3-speed auto (32RH) used 1987-2002, deemed the TF999 in older 1987-1991 YJ Wranglers

31's - 3.73
33's - 4.10
35's - 4.56
37's - 4.88

4-speed auto (42RLE) used 2003-2006, actually used longer in the JK but I'm not including JK Wranglers.

31's - 4.56, even though the factory Rubicon used 4.10 unfortunately
33's - 4.88
35's - 5.38 (Rubicon's have to change carriers to support this deep of a gear ratio)
37's - forget about it unless you get axles that support 5.89.

6-speed manual NSG370
31's 3.73 more street, 4.10 more trail or mountainous/hilly highway
33's - 4.10 more street, 4.56 more trail or mountainous/hilly highway
35's - 4.56 more street, 4.88 more trail or mountainous/ hilly highway

2.5L & 2.4L engine

5-speed AX5/NV1500

31's - 4.56/4.88 (either is good, 4.88 really helps keep speed on the highway)
33's - 4.88
35's - 5.38 if you have the axle for it, otherwise 5.13

3-speed auto 30RH

31's - 4.10
33's - 4.56
35's - 4.88

4-speed auto 42RLE

31's - 4.88
33's - 5.13 or 5.38 if you can run them.
35's+ - wouldn't even consider it.
 
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Brantley

TJ Enthusiast
Gearing properly takes experience, not just mathematical calculations.
Exactly, couldn't have said it better myself. Unfortunately, that wasn't mentioned to the thousands of viewers of that fan base so who knows how many were led incorrectly.

I'm very glad I picked my gear ratios off of some very informed guys I followed on the forums. Most of my info came from Jerry and Blaine and then anybody they had given advice to who ended up liking it (which was pretty much everybody). Glad I didn't get my advice from videos of doing math. It just isn't the same.
 
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glwood

TJ Expert
Supporting Member
Sep 16, 2016
5,474
Albuquerque, NM
Don't plan to ever go with 33" tires, but I do run 265/75-16, close enough to 32". Per your recommendation above:

4-speed auto (42RLE) used 2003-2006, actually used longer in the JK but I'm not including JK Wranglers.

31's - 4.56, even though the factory Rubicon used 4.10 unfortunately
33's - 4.88
35's - 5.38 (Rubicon's have to change carriers to support this deep of a gear ratio).
37's - forget about it unless you get axles that support 5.89.

Would 4.56 be acceptable to help mitigate the 42RLE engine lugging issue in O/D with 32" tires, or would it need to be 4.88? Most recommendations are based on 33" tires.
 
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Brantley

TJ Enthusiast
Don't plan to ever go with 33" tires, but I do run 265/75-16, close enough to 32". Per your recommendation above:

4-speed auto (42RLE) used 2003-2006, actually used longer in the JK but I'm not including JK Wranglers.

31's - 4.56, even though the factory Rubicon used 4.10 unfortunately
33's - 4.88
35's - 5.38 (Rubicon's have to change carriers to support this deep of a gear ratio).
37's - forget about it unless you get axles that support 5.89.

Would 4.56 be acceptable to help mitigate the 42RLE engine lugging issue in O/D with 32" tires, or would it need to be 4.88? Most recommendations are based on 33" tires.
I would still run 4.88 rather than drop to 4.56. I would only drop to 4.56 for 31" tires, but would go 4.88 for 32" and 33". It would perform better and the rpm would still be completely acceptable. Some people choose 5.13 for 33" tires and that would have higher RPM than 4.88 and 32". So, I'd go 4.88 all the way.
 
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glwood

TJ Expert
Supporting Member
Sep 16, 2016
5,474
Albuquerque, NM
I would still run 4.88 rather than drop to 4.56. I would only drop to 4.56 for 31" tires, but would go 4.88 for 32" and 33". It would perform better and the rpm would still be completely acceptable. Some people choose 5.13 for 33" tires and that would have higher RPM than 4.88 and 32". So, I'd go 4.88 all the way.
Gotcha, thanks.
 
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sgtjdgelinas

TJ Enthusiast
Feb 17, 2017
569
Sanford nc
Misinformation like this is probably why the guy in the JK at the daycare last week asked me if my Jeep screamed on the highway after I told him it has 4:88's, he seemed surprised when I told him I would probably go with a smaller tire size.
 
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Brantley

TJ Enthusiast
Gotcha, thanks.
Another way to look at it is that I bet if you asked around for people with 33/4.88, they probably wouldn't hesitate to swap to 32" tires if they really wanted them. I bet @Chris would still be fine with the highway rpm and would probably appreciate the tiny bit of performance gain he'd see.

Not that any of them want 32" tires, but if they did, I'm sure the owners of those setups wouldn't hesitate to run 32's on their 4.88 gears.
 
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Brantley

TJ Enthusiast
Misinformation like this is probably why the guy in the JK at the daycare last week asked me if my Jeep screamed on the highway after I told him it has 4:88's, he seemed surprised when I told him I would probably go with a smaller tire size.
Yep. You have to have a bit of knowledge to understand that these engines aren't supposed to be lugged. The goal is not to gear them so high that you drop the RPM to 2000 on the highway, resulting in no power.

JK owners have completely different engines but they seem to be afraid of RPM as well. Let it spin some, it's good for it.
 

Chris

Administrator
Staff Member
Sep 28, 2015
34,467
Salem, Oregon
I know @mrblaine would get a kick out of this.

I get really annoyed when people ask what gear ratio they should go with, and then people respond with that stupid gear ratio chart (you know the one). That gets under my skin every time.
 
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Chris

Administrator
Staff Member
Sep 28, 2015
34,467
Salem, Oregon
Another way to look at it is that I bet if you asked around for people with 33/4.88, they probably wouldn't hesitate to swap to 32" tires if they really wanted them. I bet @Chris would still be fine with the highway rpm and would probably appreciate the tiny bit of performance gain he'd see.

Not that any of them want 32" tires, but if they did, I'm sure the owners of those setups wouldn't hesitate to run 32's on their 4.88 gears.
The 4.88s work well on my Rubicon with the 42RLE transmission. But you're right, I wouldn't hesistate to put 32" tires on at all!
 
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Brantley

TJ Enthusiast
I know @mrblaine would get a kick out of this.

I get really annoyed when people ask what gear ratio they should go with, and then people respond with that stupid gear ratio chart (you know the one). That gets under my skin every time.
Oh I know the chart all too well! I think the only thing that infuriates me more than that stupid chart is either the people using them to back their theories up, or the video I linked. Just plain misinformation in general is irritating and the chart is a big part of it.

You have to go off of experience, not math. I'm so glad I never ended up using charts for myself because I would have been sorely disappointed.
 
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mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
3,393
Quail Valley, CA
Misinformation like this is probably why the guy in the JK at the daycare last week asked me if my Jeep screamed on the highway after I told him it has 4:88's, he seemed surprised when I told him I would probably go with a smaller tire size.
Why is this misinformation acceptable to learn how to mitigate and the misinformation about how bolts work meets so much resistance?
 
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mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
3,393
Quail Valley, CA
Another way to look at it is that I bet if you asked around for people with 33/4.88, they probably wouldn't hesitate to swap to 32" tires if they really wanted them. I bet @Chris would still be fine with the highway rpm and would probably appreciate the tiny bit of performance gain he'd see.

Not that any of them want 32" tires, but if they did, I'm sure the owners of those setups wouldn't hesitate to run 32's on their 4.88 gears.
I recently regeared for 32's and went with 4.88's because that is the correct ratio. Works very well. The biggest issue I have in these discussions is getting folks to understand that the 4.0 just doesn't mind running at higher RPM that the internet believes it does.
 
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Brantley

TJ Enthusiast
I recently regeared for 32's and went with 4.88's because that is the correct ratio. Works very well. The biggest issue I have in these discussions is getting folks to understand that the 4.0 just doesn't mind running at higher RPM that the internet believes it does.
Exactly. I'm not sure where the myth of "lower RPM = better" in a Jeep came from but there sure seems to be a lot of ill informed people who believe it.
 
Reactions: Chris

mrblaine

TJ Expert
Supporting Member
Nov 20, 2015
3,393
Quail Valley, CA
Oh I know the chart all too well! I think the only thing that infuriates me more than that stupid chart is either the people using them to back their theories up, or the video I linked. Just plain misinformation in general is irritating and the chart is a big part of it.

You have to go off of experience, not math. I'm so glad I never ended up using charts for myself because I would have been sorely disappointed.
I use Grimm Jeeper's chart at least once a week. My favorite is when folks tell me my numbers are wrong because their buddy has such and such set up and he is running a different RPM at 70. I have to explain there is only one variable in the math and it's simple math and the variable is the tire size. If you use 35" for your 35's, you're wrong. You need to measure from the center of the rim to the ground and then double it to get the rolling radius and plug that number in.

FYI, I have found many mistakes in every video of his I've watched that someone posts up about how to do something.
 
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Brantley

TJ Enthusiast
I use Grimm Jeeper's chart at least once a week. My favorite is when folks tell me my numbers are wrong because their buddy has such and such set up and he is running a different RPM at 70. I have to explain there is only one variable in the math and it's simple math and the variable is the tire size. If you use 35" for your 35's, you're wrong. You need to measure from the center of the rim to the ground and then double it to get the rolling radius and plug that number in.
I live on the Grimm Jeeper chart. And of course, if you put the nominal tire size, the RPM will be way off. If I put 35" tires on the 4.88/AX15 combo, it results on about 2500 at 70 and 2750 at 75. Simply isn't the case, where it actually runs about 2700 at 70 and 2900 at 75.

I typically subtract around 1-1.5" when I'm putting in the tire size on the calculator. My 31x10.50 tires are right at 29.5 inches but the 35x12.50 MT/R's on my old CJ measured right at 34" following the method you suggested. Measuring is always best and it's definitely accurate whereas going off of the written nominal tire size isn't accurate at all.
 
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