I just looked at your profile and you have a 6 speed. Your tire size says 255 but there has been no mention of if that's what you still have, have changed them or plan to change them. The great majority of experienced people on the list recommend 4.88's for 6 speeds with 35's and 5.13's for auto's with 35's. Very few recomend going deeper on gears with a stick unless you are going 37-42 on tires. Most fell 4.56's, if they're even available, wouldn't be enough of a change to make it worthwhile given the dollars gears cost.

It sounds like you are impressed by "experts". Do you let the carpenter or electrician tell you how to design your house? Because somebody is in a shop doesn't make them an expert; though they could be. Does the guy making the recomendation own a JK? How much time has he spent behind the wheel of a JK both on the highway and off road in various conditions. Driving it for a few miles after a gear install to see if it makes noise doesn't count for much in this regard. Now none, or very very few, of us are backyard mechanics when it comes to gears. I'll do damn near everything else; but not gears. But what we do bring to the table is a lot of time with the JK and we've figured out what works, what makes power, and what it's effect on fuel economy is.

Forget the charts. I haven't seen one yet that factors in the OD ratio. And they are the same charts I saw when I was regearing on old pickup in the 70's. The JK's 3.8 is a different bread of cat. Those charts made sense for a 350 Chevy with a 350 Turbo behind it but they have no relevance to us. What has relevance is what real world experience has shown works with this vehicle and or engine and tranny combos.

 

Uh, no I don't let electricians design my house, I let them decide how the wiring goes in, that's what experts are for, not buddies. I also let the carpenter decide how something made of wood is installed, not my buddies, again that's why you ask an expert. Yes my local shop owner owns and drives a JK. As for the charts, I'm not worried about OD just where the power band will be compared to where it is stock now. I know my current performance and know where I want it to be when I eventually regear, that's what the charts are for. OD doesn't figure into those numbers, that's why it isn't in the charts.

And um, no, the JK isn't some new miraculous breed of Jeep that is totally different from previous models. It's just an upgraded version with some new electronics, wider stance and beefed up parts. Still has an engine, gears, axles, tires and such like every other Jeep on the road. Gearing ratios are gearing ratios, it's just math. Doesn't matter what the vehicle is or what year it is.

Here's a fun story on some backyard mechanic friends of mine. Buddy was going to do his brakes and asked to borrow my brake tools so some other friends of his could help him install them. Sure, no problem, I'll bring them by. Dropped off the tools and mentioned to them to be sure to pre-adjust the brake shoes once the drum was back on. These "mechanics" proceeded to explain to me how they were self adjusting so you didn't need to do that. I said, "okay" and promptly left. Buddy called me back later and said he wished he'd listened to me. After they were done, he backed out of the yard and directly into a parked car in the street since he had no brakes.

 

Having been a carpenter and general contractor I never let the electricians or carpenters make the decisions you mentioned above. They follow the plans; not what they think is the best way to do it.

OK, I guess you're going to go down the freeway with the OD turned off if you have an auto or never use 6th, your OD gear, if you have a 6 speed. So I guess the existence of a .69 or .84 OD ratio and it's effect on things doesn't matter to you. Fine, your choice. Use the 40 year old charts. As a point of interest a stock Rubi, if it did not have an OD transmission would be going down the freeway a 3228 rpm at 75 mph; meaning it's 4.10 gears were way too low and something like 3.73's or even less would be more appropriate. But I forgot; I was just informed by an expert that OD doesn't matter and I shouldn't factor it in. My apologies for trying to inform an "expert".

No the JK isn't magical. But it does have something different; a relatively inappropriate engine whose design imposes limitations on it's HP and torque output. Previous Jeep engines have been biased towards low end torque. The 3.8L, being a 60 degree V6 is totally different. If you don't want to acknowledge that - well hey - you have to drive the thing - not me. But if you don't think there's a difference I've got a 6 cyl auto TJ setting next to my JK in the driveway. I challenge anybody to drive them both back to back and think different thinking shouldn't be applied in the selection of gears. What worked for the 4.0 TJ and it's trannys will not work for the 3.8 JK and it's trannys. Oh, but I forgot, they're the same except the JK is just "upgraded".

I have no clue as to what the point of your story was relative to our discussion here. Your friends are obviously fools who attempted to do a job well beyond their abilities. Perhaps you should submit them for a Darwin award.

 

X2 on the TJ thing, that's what my wife has. I'm so unimpressed with the v-6 for the JK, gears are the only thing that helped like it should. Hell my 258ci in my CJ with a 2barrel carb had more umph than my jk.

 

Oh, as far as experts, alot of 20-30yr mechanics on this forum who are now "backyard mechanics". Took me 2 1/2 months of going through so-called experts to even diagnose my TF case prob.Got more help off this forum than the knuckle heads working on my jeep.

 

I'm lost with the "ignore overdrive" philosphophy. I just don't think that's possible. Overdrive is exactly that. It is a gear in the transmission that allows the output shaft to spin at a rate greater than that of the crankshaft. That means that the output shaft is spinning above and OUT OF the engine's power band,and always will be while in overdrive. Couple overdrive with the lockup converters in the automatics, and the overdrive is enhanced even further out of the engine's power band. That means also, that the output shaft is not exposed to the the engine's optimal power capabilities. Without including overdrive in the equation, the proper final drive gear cannot be selected. This is typically why vehicles equipped from the factory with overdrive transmissions have gearsets a good bit lower than the "typical" 3 to 1 that "old school" non overdrive units had. Once in the final high gear with a 1 to 1 final ratio (or close to it considering converter slippage) the old school stuff was still in the engine's power band and was able to continue to make good power even in high gear. Of course, the "experts" will tell you that the old school stuff wasn't as economical as the new stuf with overdrive. Funny though, how a lot of that "new stuff" is still EPA rated with the same mileage ratings as some of their 30 plus year old brethern. I believe the reason for this is because of everything I discussed above. While it is true with overdrives that the engine is allowed to spin slower than the output shaft, because it is now completely out of it's optimal power band, it is way less efficient, and actually works harder while in overdrive to keep the vehicle at apeed. Of course, all of this is just one little peon's opinion.

 

Never said not to use over drive. Just didn't care about it. If I get the below correct my overdrive will be where it belongs.


Easier then asking everyone.

What gears should I get????

(new gear ratio) = (new tire size) x (old gear ratio) / (old tire size)

What will my RPMs be at X speed?

(RPM) = (MPH) x (gear ratio) x (336) / (tire size)

What is my gearing for overdrive. Having the overdrive on a 4x4 will let you cheat and go to lower gearing and still get decent highway driving but still not the "correct" gearing that you should have. Seems opinions will differ here.

(gear ratio) x (overdrive ratio) = (effective highway gear ratio)

Apply above to any vehicle knowing where you want to power band to be and your done. Don't even have to look at the "old" charts, even though they tell you the same thing. This was relevant 40 years ago and still is now. Doesn't matter the vehicle. Somebody prove me wrong.

 

Sonofa... I got dizzy reading that last post Redneck, I'm used to your one liners.

 

Oh, I know there are good smart people on here. I'm not trying to knock them. I just don't agree with blowing off people that are truly qualified and currently working on these things.

 

All I know is when we have electrical issues at our facility we call the electricians, not Bob in accounting. Granted, where I work our electricians are truly experts so maybe I'm used to a different caliber. They run our self contained power plant which could power a city.