37's on a Rubicon...

[conFUcius]

@lowmpg - I couldn’t tell if you had the XR with 4.56 gearing or a regular Rubicon with 4.10 gearing. There’s a mathematical formula to calculate what gear ratio to move to just to maintain a ‘stock’ feeling. Anyone who says you don’t have to re-gear hasn’t driven a properly geared Jeep. Could you get by without it? Sure - that’s a subjective opinion as some people prefer faster vs. slower acceleration and may be towing something vs. not. There’s science, math, and engineering behind why a re-gear should be done when increasing tire size.

I see you’re in MD so make a weekend trip to NC and @chevymitchell on here will re-gear the Jeep for you. He did mine and is very knowledgeable on this matter.

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[beast40]

Need to, no, but it makes a big difference.

 

[4xFUN]

3.6 Auto JLR with 37" KO2 D's and running 4.88's. If I were doing it again on a JL/JLR I would probably run 4.56's but on a heavier JLU/JLUR I think the 4.88's are ideal.
Absolutely necessary? Absolutely not if you already have 4.10's...BUT absolutely a huge difference!

 

[lowmpg]

@lowmpg - I couldn’t tell if you had the XR with 4.56 gearing or a regular Rubicon with 4.10 gearing. There’s a mathematical formula to calculate what gear ratio to move to just to maintain a ‘stock’ feeling. Anyone who says you don’t have to re-gear hasn’t driven a properly geared Jeep. Could you get by without it? Sure - that’s a subjective opinion as some people prefer faster vs. slower acceleration and may be towing something vs. not. There’s science, math, and engineering behind why a re-gear should be done when increasing tire size.

I see you’re in MD so make a weekend trip to NC and @chevymitchell on here will re-gear the Jeep for you. He did mine and is very knowledgeable on this matter.

Might investigate that.

 

[SouthernJL]

Well now that I viewed the tire weights, turns out my current Falken WildPeak M/Ts at 35/12.5/17 weigh about 8lbs more than the 37/12.5/17 KO2's I want to go to, so no problem with weight I guess

I live in Atlanta, elevation of about 1000 ft. At that elevation, the HP lost is almost nothing on my V6. My JLUR is a light build, with a hardtop and plastic bumpers and I don't currently take my Jeep offroad. I decided on the very light C load Ko2 37s for the looks but I went with a legit lift (2.5 in Rock Krawler) in case I decide to offroad later. I assumed I'd have to regear because I absolutely had to in my JKU when I went up to 35s. But someone on the forum mentioned that I may be shocked at not needing to after I experience the ride and they were right.

Do I notice the power loss going from the stock 33s to the 37s? Of course. You can't go bigger and heavier and not notice it. Is the loss so bad that I feel I need to spend $2k to get back 8th gear and maybe gain back 1 or 2 mpgs? Nope.

Again, you'd have to decide based on your specific situation but for my use case I'm definitely fine without the regear.

 

[landshark99]

to be honest one needs to think of why do you want go to 37s over say 33 or 35s, if it was for looks, then 4.10s will be fine. To my eye 35's and 2.5 lift look great with the jeep, for everyday driving honestly 35's are a great all around tire size, side benefit is easier for the whole family and dogs to get in and out of the jeep. The reason i went to 37s was for what that size gave me in performance, so why not use the tire size to it's max benefit, that meant regearing was the way to go, no regrets about going to 4.88s

 

[Devil6Dog]

We have been on 37's for 30k miles, recently went to 38's. We've wheeled all over the western states, multiple 1-2k mile road trips, moab twice, telluride, OR, AZ, and all over CA on the stock 4.10's. The 2.0t is still quick and fun to drive. Average mpg is ~17, not great but not terrible by any means. People get hung up on 8th gear (which me do see pretty regularly by the way), but fail to realize that no other wrangler before had such a ridiculously tall 2nd Overdrive gear to begin with. Our 7th gear is the equivalent of every other OD transmission ever offered in its highest gear. If we are talking pre-overdrive Era that is our 6th gear and yet people still drove around just fine? Couple that with the fact that our 1st gear is so low in comparison to every auto ever offered in a wrangler (a jk on 5.13's had the same crawl ratio/ 1st gear drive ratio, an early tj would need 6.72 gears to match), regearing is 100% optional in my opinion.

Thanks, this is the excuse I needed to go to 38's

 

[gato]

If I put 37x12.5x17's on my 2021 Rubicon, do I need to regear or not?

Let's see. Rubicons come in 2dr (400 lbs lighter) or 4 door.

They come with manual 6-speed or automatic 8-speed (with short 1st geat, torque converter multiplication and great gear spread).

They come with engines varying from 260 ftlbs to 470 ftlbs.

They come with gearing between 3.73 and 4.88 (with 4.10s and 4.56 being more typical).

And 37x12.5 tires vary in weight from 65 lbs to 95 lbs.

Also, some drivers like to cruise on the highway at 55MPH while other are closer to 85MPH.

If you want a more helpful answer, you could include your choice of the above options.

I can tell you that on my heavy 4-door with 69lbs KO2 37x12.5 and my 2.0T engine and A8 with my preferred cruising speed of ˜75MPH, I feel no need to regear. 8th gear shows just fine at 75MPH and take off/offroad power is great. But that is my situation. What is yours?

 

[lowmpg]

Let's see. Rubicons come in 2dr (400 lbs lighter) or 4 door.

They come with manual 6-speed or automatic 8-speed (with short 1st geat, torque converter multiplication and great gear spread).

They come with engines varying from 260 ftlbs to 470 ftlbs.

They come with gearing between 3.73 and 4.88 (with 4.10s and 4.56 being more typical).

And 37x12.5 tires vary in weight from 65 lbs to 95 lbs.

Also, some drivers like to cruise on the highway at 55MPH while other are closer to 85MPH.

If you want a more helpful answer, you could include your choice of the above options.

I can tell you that on my heavy 4-door with 69lbs KO2 37x12.5 and my 2.0T engine and A8 with my preferred cruising speed of ˜75MPH, I feel no need to regear. 8th gear shows just fine at 75MPH and take off/offroad power is great. But that is my situation. What is yours?

What was the point of the beginning of your response, given EVERY other responder seemed to get the question. So I'll put this on ignore.

to be honest one needs to think of why do you want go to 37s over say 33 or 35s, if it was for looks, then 4.10s will be fine. To my eye 35's and 2.5 lift look great with the jeep, for everyday driving honestly 35's are a great all around tire size, side benefit is easier for the whole family and dogs to get in and out of the jeep. The reason i went to 37s was for what that size gave me in performance, so want not use the tire size to it's max benefit, that meant regearing was the way to go, no regrets about going to 4.88s

Thanks for this!

 

[Zandcwhite]

@lowmpg - I couldn’t tell if you had the XR with 4.56 gearing or a regular Rubicon with 4.10 gearing. There’s a mathematical formula to calculate what gear ratio to move to just to maintain a ‘stock’ feeling. Anyone who says you don’t have to re-gear hasn’t driven a properly geared Jeep. Could you get by without it? Sure - that’s a subjective opinion as some people prefer faster vs. slower acceleration and may be towing something vs. not. There’s science, math, and engineering behind why a re-gear should be done when increasing tire size.

I see you’re in MD so make a weekend trip to NC and @chevymitchell on here will re-gear the Jeep for you. He did mine and is very knowledgeable on this matter.

This type of response assumes that the factory gearing was perfect and that every other Jeep ever made with an automatic was either grossly over geared or grossly under geared. The science and math just proves my point. We can look at the late model JK as the most relevant example as many of you are too young to remember the terrible autos we used to drive without problems. 3.59 1st gear and .83 top gear a JK had a crawl ratio of 58.9 and a final drive ratio in od of 3.4. Let's assume those are acceptable numbers for stock 31's. A 37 is 20% larger so would need 4.92 gears to keep the stock ratio, let's say 5.13's for a bump in performance over stock for a jk going to 37's (most didn't go that low as the single Overdrive would leave the rpms kinda high at freeway speed). Our 1st gear is so much lower that it is equivalent to a jk with 5.13's in crawl ratio and 1st gear high range (5.13x3.59is actually equal to our 3.92 gears x 4.7). We are literally geared lower than a jk on 5.13's from the factory. Thanks to our double Overdrive at the same time we are equal to a JK geared 3.27 when in 8th gear. That is the math and science of it. Our 8 speed auto is so good gearing is almost irrelevant. Sure you can simply compare the percentage of tire size change and assume that you need an equal gear ratio change but that ignores the history of Jeeps and what has worked well for decades. The engine doesn't know if the gearing comes from your axles or the transmission downshifting, not does it care a given load will require a given output from the engine which will be accomplished via a given rpm. That rpm can come from 5.38's in 8th gear, 4.29's in 7th, or 3.60's in 6th gear. The engine doesn't know or care. That's the math and science of it. With an 8 speed auto any gearing can work. For someone who does multiple 1-2k mile road trips every year, I'd rather see 2100 rpm in 8th gear at 85mph or 2650 rpm in 7th when the load demands than 2500 rpm in 8th with 4.88's and 3150 rpm in 7th on 38's so I'll stick with 4.10's and 38's. Gearing down to the point that 8th gear now matches the stock gearing in 7th gear is illogical to me (5.13 gears are identical in 8th gear to 4.10's in 7th...math). Unless you feel like 1st gear is too tall, there's no reason to regear.

 

[word302]

This type of response assumes that the factory gearing was perfect and that every other Jeep ever made with an automatic was either grossly over geared or grossly under geared. The science and math just proves my point. We can look at the late model JK as the most relevant example as many of you are too young to remember the terrible autos we used to drive without problems. 3.59 1st gear and .83 top gear a JK had a crawl ratio of 58.9 and a final drive ratio in od of 3.4. Let's assume those are acceptable numbers for stock 31's. A 37 is 20% larger so would need 4.92 gears to keep the stock ratio, let's say 5.13's for a bump in performance over stock for a jk going to 37's (most didn't go that low as the single Overdrive would leave the rpms kinda high at freeway speed). Our 1st gear is so much lower that it is equivalent to a jk with 5.13's in crawl ratio and 1st gear high range (5.13x3.59is actually equal to our 3.92 gears x 4.7). We are literally geared lower than a jk on 5.13's from the factory. Thanks to our double Overdrive at the same time we are equal to a JK geared 3.27 when in 8th gear. That is the math and science of it. Our 8 speed auto is so good gearing is almost irrelevant. Sure you can simply compare the percentage of tire size change and assume that you need an equal gear ratio change but that ignores the history of Jeeps and what has worked well for decades. The engine doesn't know if the gearing comes from your axles or the transmission downshifting, not does it care a given load will require a given output from the engine which will be accomplished via a given rpm. That rpm can come from 5.38's in 8th gear, 4.29's in 7th, or 3.60's in 6th gear. The engine doesn't know or care. That's the math and science of it. With an 8 speed auto any gearing can work. For someone who does multiple 1-2k mile road trips every year, I'd rather see 2100 rpm in 8th gear at 85mph or 2650 rpm in 7th when the load demands than 2500 rpm in 8th with 4.88's and 3150 rpm in 7th on 38's so I'll stick with 4.10's and 38's. Gearing down to the point that 8th gear now matches the stock gearing in 7th gear is illogical to me (5.13 gears are identical in 8th gear to 4.10's in 7th...math). Unless you feel like 1st gear is too tall, there's no reason to regear.

I don’t disagree, but you need to add rolling mass into the equation as well. Yes the 2.0 fair a little better but 38s on 4.10s would definitely not work for my use case even if I had the 4 banger and auto (I don’t, manual 3.6 for me so my opinion here is pretty irrelevant). And yes, Jeeps have been geared horribly for years, though the stock hearing on the manual sports and Saharas is unforgivable.

 

[conFUcius]

This type of response assumes that the factory gearing was perfect and that every other Jeep ever made with an automatic was either grossly over geared or grossly under geared. The science and math just proves my point. We can look at the late model JK as the most relevant example as many of you are too young to remember the terrible autos we used to drive without problems. 3.59 1st gear and .83 top gear a JK had a crawl ratio of 58.9 and a final drive ratio in od of 3.4. Let's assume those are acceptable numbers for stock 31's. A 37 is 20% larger so would need 4.92 gears to keep the stock ratio, let's say 5.13's for a bump in performance over stock for a jk going to 37's (most didn't go that low as the single Overdrive would leave the rpms kinda high at freeway speed). Our 1st gear is so much lower that it is equivalent to a jk with 5.13's in crawl ratio and 1st gear high range (5.13x3.59is actually equal to our 3.92 gears x 4.7). We are literally geared lower than a jk on 5.13's from the factory. Thanks to our double Overdrive at the same time we are equal to a JK geared 3.27 when in 8th gear. That is the math and science of it. Our 8 speed auto is so good gearing is almost irrelevant. Sure you can simply compare the percentage of tire size change and assume that you need an equal gear ratio change but that ignores the history of Jeeps and what has worked well for decades. The engine doesn't know if the gearing comes from your axles or the transmission downshifting, not does it care a given load will require a given output from the engine which will be accomplished via a given rpm. That rpm can come from 5.38's in 8th gear, 4.29's in 7th, or 3.60's in 6th gear. The engine doesn't know or care. That's the math and science of it. With an 8 speed auto any gearing can work. For someone who does multiple 1-2k mile road trips every year, I'd rather see 2100 rpm in 8th gear at 85mph or 2650 rpm in 7th when the load demands than 2500 rpm in 8th with 4.88's and 3150 rpm in 7th on 38's so I'll stick with 4.10's and 38's. Gearing down to the point that 8th gear now matches the stock gearing in 7th gear is illogical to me (5.13 gears are identical in 8th gear to 4.10's in 7th...math). Unless you feel like 1st gear is too tall, there's no reason to regear.

Clearly the manufacturer thought the OEM gear ratio was sufficient to produce it at that ratio. I believe the formula is [(Desired Tire Size)(OEM Gear Ratio)]/(OEM Tire Size) to preserve the OEM ‘feel’ from factory… not sure how many people follow the straight percentages you stated. That’s why people re-gear because you lose it when you go to a bigger tire. Obviously, you’re biased because you believe Jeeps have too short of gearing from factory which is why you’re content slapping 37s on yours and not re-gearing. For those who follow the math - like @chevymitchell - you’d re-gear to 4.88s. My point is that you can’t deny that slapping 37s on a Jeep without a re-gear doesn’t impact the available acceleration off the line - the OP should try driving one that is re-geared appropriately to see if it matches the OEM feel… that’s how the OP will be able to determine if a re-gear is necessary.

 

[Zandcwhite]

Clearly the manufacturer thought the OEM gear ratio was sufficient to produce it at that ratio. I believe the formula is [(Desired Tire Size)(OEM Gear Ratio)]/(OEM Tire Size) to preserve the OEM ‘feel’ from factory… not sure how many people follow the straight percentages you stated. That’s why people re-gear because you lose it when you go to a bigger tire. Obviously, you’re biased because you believe Jeeps have too short of gearing from factory which is why you’re content slapping 37s on yours and not re-gearing. For those who follow the math - like @chevymitchell - you’d re-gear to 4.88s. My point is that you can’t deny that slapping 37s on a Jeep without a re-gear doesn’t impact the available acceleration off the line - try driving one that is re-geared appropriately to see if it matches the OEM feel.

With the superchips tune, even with 38’s/armor/lift/winch/and ~300lbs of tools and recovery gear in the back and 350lbs worth of passenger it still does 0-60 in 7.0 seconds (still faster than any previous wrangler model). Sure I could spend $2k to make it quicker and cost myself ~500 rpm extra on the freeway, but does it need to be any quicker?

 

[word302]

Clearly the manufacturer thought the OEM gear ratio was sufficient to produce it at that ratio. I believe the formula is [(Desired Tire Size)(OEM Gear Ratio)]/(OEM Tire Size) to preserve the OEM ‘feel’ from factory… not sure how many people follow the straight percentages you stated. That’s why people re-gear because you lose it when you go to a bigger tire. Obviously, you’re biased because you believe Jeeps have too short of gearing from factory which is why you’re content slapping 37s on yours and not re-gearing. For those who follow the math - like @chevymitchell - you’d re-gear to 4.88s. My point is that you can’t deny that slapping 37s on a Jeep without a re-gear doesn’t impact the available acceleration off the line - the OP should try driving one that is re-geared appropriately to see if it matches the OEM feel… that’s how the OP will be able to determine if a re-gear is necessary.

Again, most if the factory gearing is just ok. The manual non-rubicon hearing from the factory is atrocious. Borderline criminal. There’s also another huge piece to the puzzle; transfer case gearing. If you’re on 4.10 gears with the 2.0 or diesel you could easily live with the factory gearing and 35” tires. Rolling mass plays a huge roll as well. It’s not as simple as the gear calculators would have you think. There are way too many variables.

 

[conFUcius]

With the superchips tune, even with 38’s/armor/lift/winch/and ~300lbs of tools and recovery gear in the back and 350lbs worth of passenger it still does 0-60 in 7.0 seconds (still faster than any previous wrangler model). Sure I could spend $2k to make it quicker and cost myself ~500 rpm extra on the freeway, but does it need to be any quicker?

If you want the Jeep to feel ‘stock’ with all that stuff… then, yes. If you don’t, then no. At the end of the day, we’re all biased to justify why we mod (or don’t mod) the Jeeps the way we do (or don’t) - few are willing to admit it was a waste of money to buy certain mods.

Have you driven a Jeep that’s been re-geared to bring the gear ratio close to the feel from factory? If so, and you say it’s not worth it, good for you. Most people don’t and just try to justify to themself not to spend the money and think they can get by without having not even tried it… I’m just letting OP know that.

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