5.13 gears
JK Junkie
Joined: Jul 2008
Posts: 3,017
Likes: 0
From: Lakewood, OH
This is all pretty telling, and the price tag on additional rpms on gear sets people are considering are generally minimal. From 4.56 to 5.38s there is less than an 18% difference, and that's well beyond what most people have their choices narrowed down to. On the automatics, you are battling with an under powered engine and trying to minimize kick downs and unlocking the converter. Erring to the steeper gear set is in order as the minimal amount of additional rpms will offset any loss in fuel economy by not kicking down as much.
JK Freak
Joined: Jan 2010
Posts: 726
Likes: 0
From: Charlotte, NC
I just put 5.13 gears into my Auto JK riding on 33'' tires, and it doesn't feel overgeared. My RPM's are around 2700 at 70, which is as fast as I normally drive. I did a 350 mile through some pretty hilly terrain this week and it didn't downshift once. It's like the hills wern't even there. My MPG went up by 1 MPG during the break in period, and it may go higher. Right now, 65-70 MPH yields about 17 MPG. I'm getting 15 in the city. Those are hand calculated numbers, which are 1 MPG lower then the computer readout.
JK Freak
Joined: Jul 2010
Posts: 889
Likes: 2
From: Fargo, ND
i don't know how some are so adamant about arguing facts. as planman stated 2+2=4 all day, everyday. with that said, my drive home for the holidays xmas eve into a 30 mph headwind on my 08 2dr X auto had me averaging 11.5 mpg on a 200 mile trip. OD off the entire way and still found it a struggle maintaining speed going up any long grade into the wind. with OD off at 75 mph i'm running about 3100 rpm and i think that even going 5.13 on the stock 32" BFGs and being able to utilize OD wouldn't be out of question until i'm due for new rubber and moving into 35's. the 3.8 stinks lugging along at 2k unless its dead calm and flat as a pancake.
long story short, planman has it spelled out. its science, not witchcraft.
long story short, planman has it spelled out. its science, not witchcraft.
JK Super Freak
Joined: Apr 2007
Posts: 1,258
Likes: 1
From: Prosper, TX
Science, not witchcraft is correct. The science of it is that from the factory, Jeep engineers set these up to pull freeway speeds at the 1900 RPM to the 2200 RPM range, with the Rubi 6 speed as beeing the oddball coming in just above 2500 RPM. Even the Rubi auto comes in at 2080. Look at the numbers for a stock JK and you will see what I am talking about. If the stock motor/tranny combos were inadequate in that range, why would JEEP ENGINEERS, the Jeep experts set the vehicles up for that. They did it for several reasons. Fuel economy was one. To get a certain range you have to test across a fleet and those very same Jeep engineers determined that there was enough power and economy to gear our JKs like they did. So like you said, it is science.
So the science of it is, you have the JK X (sport) coming in at 1900-2200 RPM for both the auto and stick. You have the Sahara auto and stick coming in at the 1900-2200 RPM range. You have the Rubi auto coming in at the 2080 RPM range and the lone dissenter is the Rubi 6 SP at 2500. Then you can say the same thing about the JKU. That leaves you with essentially 10 JK models in the 1900 to 2200 RPM range and just 2 at the 2500 RPM range. So, again the JEEP EGINEERS, the ones who designed these things, the expert teams of engineers who say 8 models to 2 that 1900-2200 is the best range. Those are the facts. So then, according to many on here, they are wrong. Keep in mind, they do this for a living, we do not in the majority of cases. So again, we have the many non experts saying that the people who get paid and do this for a living are wrong. I will side with the experts here. Those are the ones who get paid to design vehicles. That is what they do.
Lesson #2. For those that say the JK doesn't have enough HP to cruise easily without shifting gears constantly and overheating juice trannys, this part is for you. Some with like to throw out the low HP numbers of the 3.8 but that is where you err. It is not the HP you should be concerned with. It is the torque. Torque is the pulling power that does the work. Look at the torque curve of our motor. It's a pretty flat torque curve for the most part but look at the 2000 RPM range. It has about 169 ft-lbs stock. Now those Jeep engineer teams felt this is a good range for optimum cruising for a reason. Not 2500, or all Jeeps would be geared like that. Many of us also will then put a tuner and other mods on to further boost that. So let's just say you are running a Hyptertech premium tune and that torque goes up to 190 from the 167. Throw on a CAI and free flowing exhaust and you may be able to pull 200 ft-lbs. No power house by any means but those additions will be enough to overcome the added rolling resistance of a lift and the added weight of tires and other things. (BTW, the Jeep engineer teams wouldn't allow for a tranny hunting gears and overheating and those same experts felt they were not having those problems with their set ups. If they had, they wouldn't have geared our jeeps like they did)
So yes, it is science but you have to use the correct figures. Torque, not HP.
I wanted to post this up earlier but was out of town and it's hard to do on an iphone.
BTW, just yet another cross state trip with the JKU turning 1700-2200 rpm with no gear hunting and tranny overheating like some here like to claim. An ya, it has enough TORQUE stock to maintain that ride without issue.
So yes, it is science, but you have to use the correct science. Just like the Jeep Engineer teams who felt that 1900-2200 rpm is a good range for us and that our jeeps do in fact have the power to pull (again, torque, not HP) and that is why those experts chose to gear our Jeeps like they did on 8/10 models.
Enzo Ferrari was once quoted as saying "Horsepower sells cars, torque wins races".
So the science of it is, you have the JK X (sport) coming in at 1900-2200 RPM for both the auto and stick. You have the Sahara auto and stick coming in at the 1900-2200 RPM range. You have the Rubi auto coming in at the 2080 RPM range and the lone dissenter is the Rubi 6 SP at 2500. Then you can say the same thing about the JKU. That leaves you with essentially 10 JK models in the 1900 to 2200 RPM range and just 2 at the 2500 RPM range. So, again the JEEP EGINEERS, the ones who designed these things, the expert teams of engineers who say 8 models to 2 that 1900-2200 is the best range. Those are the facts. So then, according to many on here, they are wrong. Keep in mind, they do this for a living, we do not in the majority of cases. So again, we have the many non experts saying that the people who get paid and do this for a living are wrong. I will side with the experts here. Those are the ones who get paid to design vehicles. That is what they do.
Lesson #2. For those that say the JK doesn't have enough HP to cruise easily without shifting gears constantly and overheating juice trannys, this part is for you. Some with like to throw out the low HP numbers of the 3.8 but that is where you err. It is not the HP you should be concerned with. It is the torque. Torque is the pulling power that does the work. Look at the torque curve of our motor. It's a pretty flat torque curve for the most part but look at the 2000 RPM range. It has about 169 ft-lbs stock. Now those Jeep engineer teams felt this is a good range for optimum cruising for a reason. Not 2500, or all Jeeps would be geared like that. Many of us also will then put a tuner and other mods on to further boost that. So let's just say you are running a Hyptertech premium tune and that torque goes up to 190 from the 167. Throw on a CAI and free flowing exhaust and you may be able to pull 200 ft-lbs. No power house by any means but those additions will be enough to overcome the added rolling resistance of a lift and the added weight of tires and other things. (BTW, the Jeep engineer teams wouldn't allow for a tranny hunting gears and overheating and those same experts felt they were not having those problems with their set ups. If they had, they wouldn't have geared our jeeps like they did)
So yes, it is science but you have to use the correct figures. Torque, not HP.
I wanted to post this up earlier but was out of town and it's hard to do on an iphone.
BTW, just yet another cross state trip with the JKU turning 1700-2200 rpm with no gear hunting and tranny overheating like some here like to claim. An ya, it has enough TORQUE stock to maintain that ride without issue.
So yes, it is science, but you have to use the correct science. Just like the Jeep Engineer teams who felt that 1900-2200 rpm is a good range for us and that our jeeps do in fact have the power to pull (again, torque, not HP) and that is why those experts chose to gear our Jeeps like they did on 8/10 models.
Enzo Ferrari was once quoted as saying "Horsepower sells cars, torque wins races".
Last edited by CJ7nvrstk; Dec 27, 2011 at 02:07 AM.
JK Junkie
Joined: Jul 2008
Posts: 3,017
Likes: 0
From: Lakewood, OH
Vehicles aren't manufactured on the whims of Engineers alone. You have marketing people aiming for EPA estimates and business people wanting to use bin parts to reduce costs. A lot of strings pulling at what creates the final product, and you don't need to look any further than Daimler looking for a suitor, and the only one they could find was Cerberus, a private equity firm with no real automotive experience. Chrysler was leveraging their brands to produce sales and their only success story was the Hemi, which was soon to be thwarted by rising fuel costs.
EPA estimates are just that, estimates. If you want mass market appeal, you need to have some decent numbers on the window sticker. EPA estimates are definitely a marketing target, and through 2007 they weren't even close to a real world measure of economy. For instance highway fuel economy was estimated using an average of 48mph and a top speed of 60mph. If engineers are driving that bus, they are aiming at real world results at 70mph and a sustainable amount of overhead so the vehicle doesn't need to kick down for moderate grades.
Horsepower is simply torque applied over time. They will always have a direct relationship. Horsepower = (Torque x RPM) / 5252. Torque is like a hammer, determining the rate of acceleration while horsepower is like a come along which determines a vehicles ability to maintain speed. Torque is certainly nice to have, but if I hit peak torque at 1200 rpm I would need some crazy numerically low gear set to apply it at highway speeds, while also having diminishing returns by increasing rpm.
Lifting a vehicle and applying larger tires has many things that increase it's need for power to maintain cruise speeds. Increased drag, Increased frontal area of vehicle, Additional weight, Increased tire rolling resistance, Increased speed effect of tire rolling resistance and Increased rolling load. Moving from a 32" tire to a 35" tire with a lift can require 15 or more additional horsepower to maintain highway speed. Note, we cannot use torque to determine what is needed to maintain speed because there is no element of time. Torque can move our 2 ton vehicle, but you need the element of time to determine if it's going to move an inch, a foot or a mile in a minute. The easiest way to get the additional power is by increasing rpms, be it by downshifting, turning OD off or running a numerically higher gear set.
Certainly there is a very small minority who think you can get this done running at a lower rpm. My personal experience, which very much coincides with that of planman, has demonstrated that I can't maintain 70mph on a grade as insignificant as an overpass with 35s at much less than 2300 rpm, operating at 600' above sea level. Steeper grades with my 35s while still not getting above 3% and I need to downshift. Most of all, I see little to no difference in efficiency running at 2700 as opposed to 2300, allowing the throttle blade to regulate power. Now move to an automatic transmission where you have the extra caveats of unlocking the converter then flashing your rpms as much as 50% higher, and you can quickly realize it's not going to take a lot of occurrences to quickly eat up any benefit of lower rpm cruise speeds.
EPA estimates are just that, estimates. If you want mass market appeal, you need to have some decent numbers on the window sticker. EPA estimates are definitely a marketing target, and through 2007 they weren't even close to a real world measure of economy. For instance highway fuel economy was estimated using an average of 48mph and a top speed of 60mph. If engineers are driving that bus, they are aiming at real world results at 70mph and a sustainable amount of overhead so the vehicle doesn't need to kick down for moderate grades.
Horsepower is simply torque applied over time. They will always have a direct relationship. Horsepower = (Torque x RPM) / 5252. Torque is like a hammer, determining the rate of acceleration while horsepower is like a come along which determines a vehicles ability to maintain speed. Torque is certainly nice to have, but if I hit peak torque at 1200 rpm I would need some crazy numerically low gear set to apply it at highway speeds, while also having diminishing returns by increasing rpm.
Lifting a vehicle and applying larger tires has many things that increase it's need for power to maintain cruise speeds. Increased drag, Increased frontal area of vehicle, Additional weight, Increased tire rolling resistance, Increased speed effect of tire rolling resistance and Increased rolling load. Moving from a 32" tire to a 35" tire with a lift can require 15 or more additional horsepower to maintain highway speed. Note, we cannot use torque to determine what is needed to maintain speed because there is no element of time. Torque can move our 2 ton vehicle, but you need the element of time to determine if it's going to move an inch, a foot or a mile in a minute. The easiest way to get the additional power is by increasing rpms, be it by downshifting, turning OD off or running a numerically higher gear set.
Certainly there is a very small minority who think you can get this done running at a lower rpm. My personal experience, which very much coincides with that of planman, has demonstrated that I can't maintain 70mph on a grade as insignificant as an overpass with 35s at much less than 2300 rpm, operating at 600' above sea level. Steeper grades with my 35s while still not getting above 3% and I need to downshift. Most of all, I see little to no difference in efficiency running at 2700 as opposed to 2300, allowing the throttle blade to regulate power. Now move to an automatic transmission where you have the extra caveats of unlocking the converter then flashing your rpms as much as 50% higher, and you can quickly realize it's not going to take a lot of occurrences to quickly eat up any benefit of lower rpm cruise speeds.
JK Freak
Joined: Jan 2010
Posts: 969
Likes: 0
From: N. Prov. , RI
Originally Posted by JPop
Vehicles aren't manufactured on the whims of Engineers alone. You have marketing people aiming for EPA estimates and business people wanting to use bin parts to reduce costs. A lot of strings pulling at what creates the final product, and you don't need to look any further than Daimler looking for a suitor, and the only one they could find was Cerberus, a private equity firm with no real automotive experience. Chrysler was leveraging their brands to produce sales and their only success story was the Hemi, which was soon to be thwarted by rising fuel costs.
EPA estimates are just that, estimates. If you want mass market appeal, you need to have some decent numbers on the window sticker. EPA estimates are definitely a marketing target, and through 2007 they weren't even close to a real world measure of economy. For instance highway fuel economy was estimated using an average of 48mph and a top speed of 60mph. If engineers are driving that bus, they are aiming at real world results at 70mph and a sustainable amount of overhead so the vehicle doesn't need to kick down for moderate grades.
Horsepower is simply torque applied over time. They will always have a direct relationship. Horsepower = (Torque x RPM) / 5252. Torque is like a hammer, determining the rate of acceleration while horsepower is like a come along which determines a vehicles ability to maintain speed. Torque is certainly nice to have, but if I hit peak torque at 1200 rpm I would need some crazy numerically low gear set to apply it at highway speeds, while also having diminishing returns by increasing rpm.
Lifting a vehicle and applying larger tires has many things that increase it's need for power to maintain cruise speeds. Increased drag, Increased frontal area of vehicle, Additional weight, Increased tire rolling resistance, Increased speed effect of tire rolling resistance and Increased rolling load. Moving from a 32" tire to a 35" tire with a lift can require 15 or more additional horsepower to maintain highway speed. Note, we cannot use torque to determine what is needed to maintain speed because there is no element of time. Torque can move our 2 ton vehicle, but you need the element of time to determine if it's going to move an inch, a foot or a mile in a minute. The easiest way to get the additional power is by increasing rpms, be it by downshifting, turning OD off or running a numerically higher gear set.
Certainly there is a very small minority who think you can get this done running at a lower rpm. My personal experience, which very much coincides with that of planman, has demonstrated that I can't maintain 70mph on a grade as insignificant as an overpass with 35s at much less than 2300 rpm, operating at 600' above sea level. Steeper grades with my 35s while still not getting above 3% and I need to downshift. Most of all, I see little to no difference in efficiency running at 2700 as opposed to 2300, allowing the throttle blade to regulate power. Now move to an automatic transmission where you have the extra caveats of unlocking the converter then flashing your rpms as much as 50% higher, and you can quickly realize it's not going to take a lot of occurrences to quickly eat up any benefit of lower rpm cruise speeds.
EPA estimates are just that, estimates. If you want mass market appeal, you need to have some decent numbers on the window sticker. EPA estimates are definitely a marketing target, and through 2007 they weren't even close to a real world measure of economy. For instance highway fuel economy was estimated using an average of 48mph and a top speed of 60mph. If engineers are driving that bus, they are aiming at real world results at 70mph and a sustainable amount of overhead so the vehicle doesn't need to kick down for moderate grades.
Horsepower is simply torque applied over time. They will always have a direct relationship. Horsepower = (Torque x RPM) / 5252. Torque is like a hammer, determining the rate of acceleration while horsepower is like a come along which determines a vehicles ability to maintain speed. Torque is certainly nice to have, but if I hit peak torque at 1200 rpm I would need some crazy numerically low gear set to apply it at highway speeds, while also having diminishing returns by increasing rpm.
Lifting a vehicle and applying larger tires has many things that increase it's need for power to maintain cruise speeds. Increased drag, Increased frontal area of vehicle, Additional weight, Increased tire rolling resistance, Increased speed effect of tire rolling resistance and Increased rolling load. Moving from a 32" tire to a 35" tire with a lift can require 15 or more additional horsepower to maintain highway speed. Note, we cannot use torque to determine what is needed to maintain speed because there is no element of time. Torque can move our 2 ton vehicle, but you need the element of time to determine if it's going to move an inch, a foot or a mile in a minute. The easiest way to get the additional power is by increasing rpms, be it by downshifting, turning OD off or running a numerically higher gear set.
Certainly there is a very small minority who think you can get this done running at a lower rpm. My personal experience, which very much coincides with that of planman, has demonstrated that I can't maintain 70mph on a grade as insignificant as an overpass with 35s at much less than 2300 rpm, operating at 600' above sea level. Steeper grades with my 35s while still not getting above 3% and I need to downshift. Most of all, I see little to no difference in efficiency running at 2700 as opposed to 2300, allowing the throttle blade to regulate power. Now move to an automatic transmission where you have the extra caveats of unlocking the converter then flashing your rpms as much as 50% higher, and you can quickly realize it's not going to take a lot of occurrences to quickly eat up any benefit of lower rpm cruise speeds.
Cheers!
JK Freak
Joined: Jan 2010
Posts: 726
Likes: 0
From: Charlotte, NC
My parents had a Town and Country with the same engine our JK's have. When I was considering a regear, I did some observations on it's highway performance. It seems that it ran similar RPM's as our JK's do at the same speeds. That is to say that at 70, both would turn around 2000 RPM's. However, the Town and Country almost never had to downshift. It cruised right along at 2000 RPM's on the highway without an issue. Perhaps the gearing in the JK's is appropriate for the engine, but not the aerodynamics. On paper, it should have been perfect, but in application, especially after a lift and new tires, it was downright aweful.
IIRC, the new 2012 auto's are geared lower in the transmission, and have a higher output engine. If nothing was wrong before, why lower the ratio after adding the Pentastar? My parents just got a new Pentastar powered Caravan (equivelent to the Town and Country) and it's RPM levels are the same as with their old T&C. I think this showes that Jeep did see an issue with the old 07-11 JK driveline configuration, and took steps to correct it.
IIRC, the new 2012 auto's are geared lower in the transmission, and have a higher output engine. If nothing was wrong before, why lower the ratio after adding the Pentastar? My parents just got a new Pentastar powered Caravan (equivelent to the Town and Country) and it's RPM levels are the same as with their old T&C. I think this showes that Jeep did see an issue with the old 07-11 JK driveline configuration, and took steps to correct it.
JK Enthusiast
Joined: Nov 2011
Posts: 190
Likes: 0
From: Richmond Virginia
Wow I think we have 1 out of a bizillion here a person who trust the cars companies!!! I can assure you that engineers have the last say in anything related to the American car industry. I think getting our ass kick by foreign car companies over the past 30 yrs and that little issues (what was it) oh yea that bail out thingy is a pretty good indication of the state of affairs. But that's another thread I think! I'm going with J-Pops and Planman and the others who obviously know what their doing when it comes to the mods on my Jeep.