Originally Posted by JeepLab

Ok, ive just gone thru the ringer with gears. Here’s what Ive learned. Maybe it will save someone else a pile of time and money.
There is a LOT of misinformation on the internet about gears. You really have to ignore it all. People use this chart to understand what gearing is correct, but I disagree with this chart on a few points.



Mistake 1. You don’t really know your tire size. I have 37x12.50R17s on Black Betty right? They are not 37 inches tall. I just EXTENSIVELY tested the calibration of my system because I couldn’t figure out why I hated the new gearing so much. Turns out that my truck has to be calibrated as if its tires are 35.5 inches tall. And even there, it was off by about a mile vs. the gps for each 110 miles traveled. You need good distance to really test this, as you are talking about feet difference if your off by an inch. It takes a road trip to make sure its perfect.
You rubicon owners who think you have 32s… you have 31s.

"No ones tires are the total height, everyone knows that, dummy." – Yes I knew it too. But my brain figured that under load, the tire would bulge to the sides and top. So the circumference (length around the outside of a circle) of the tire, should be the same distance per revolution as a 37 inch tire would be. The way I figured it, the tire dosent wrinkle, you have to travel the full distance of the outside revolution and thus, even tho the tire measures 35.40 inches tall, all the bulged sections at the sides and top are still tread that has to pass for a revolution.

Suffices to say. I was wrong. All the extra math I was doing means nothing. BB was only properly calibrated once I dialed her in with her tire size as the standard height. Straight from bottom to top. Totally over thought it.


Mistake 1 – selected gears based on my “imagined” tire size. LOL. When using that chart realize that your tire size is not the size you bought.

Mistake 2 – Chasing the crusing gear. With 4.10s, BB would cruise at 80 mph at 2500 rpm. Great right? I couldn’t figure out why I was losing soooo much speed with the new gears. I was expecting the difference to be few hundred rpm. Well Im here to tell you, 400 RPM is a lot. But the bigger problem was that I was calibrated correctly after the gear change, and I WASN’T before. So Im driving around thinking I have 37s on, calibrated for 37s, speedo responding like I had 37s….with 35.5s on.

Point is, I wasn’t doing 80 at 2500 rpm. So the draw back I experienced was compounded with the realization that the speedo has been lying to me for 6 months. Not by a lot, but enough to shock me when the 4.88s went in.

Keep in mind, Tires wear, and everyone runs at different PSI, My KM2s have about 4k miles on them, and im not sure how much I could have lost, but realize that we are talking about calibration, that relys on a rubber component that wears as you drive. So your gearing and speedo calibration is going to increase as the tires get smaller. You have to study your own truck to know where to start in order to use the charts correctly.

Let me know if im overthinking again.

Mistake 3 – being afraid of the yellow section of the chart. – Everything they say about the yellow section is negative. Minimum acceptable for daily driving. Maybe that’s true without the supercharger. (just realized that’s the xfactor here) but outside of that, the yellow section allows you to cruise at high speed on the highway.

If you don’t drive on the highway, fine. I don’t do it a lot, but when I do, Its long distance. And having to hold the gas pedal down further to maintain 2500 rpm to do 70 mph is taxing. For rubicon owners, especially in the north east where highways move at 80mph, You have a 4 to 1 transfer case, so when you go into 4 low off road, your truck makes up for this Yellow level of gearing, So you have the option to have a higher cruise speed on the road, without really giving up anything off road.

Ross from Ripp supercharger once warned me not to regear for this exact reason. I didn’t listen to him and he was right.

Mistake 4 – reading forums. – Yes, ill admit it. I was influenced by forums stating that 37s require 4.88s. I’ve realized that any idiot can open a forum account and blab about whatever they want. Valuable lesson, as a lot of my jeep knowledge comes from others and tons of research. When it comes to gears, ignore the forums. You have to buy them, and spend to install them before you ever find out if you’ll like them. Its invasive and risky. A lot can go wrong. So before you take out 4.10s think long and hard about it.

IF you have 3.21s and want to go to larger tires… Consider 4.10s or Yukon makes them as 4.11s They are the most expensive gear, but don’t let that scare you. Doing this twice is far more expensive.

Keep in mind, lots of people just repeat the garbage that others said without actually having that gear installed and lived with it. There is accurate information on the internet also, but for everything I read that gave the correct information I found 20 other posts that disagreed and gave bad information. Not everyone who talks like an authority is an authority. You need to check post count, and refrences.

Mistake 5 – Not realizing that manual trans is far different from auto for gearing requirements.

Many of the people yapping online have autos. This makes a huge difference. Autos need new gear more than manuals. Because your pretty much on autopilot and at its mercy to select which gear to drive in. Your also confusing it with larger tires. People regear autos and it makes a big difference because the get better acceleration because they may be stuck in a higher gear as the cpu is deciding which gear is right.

In a Manual trans, if 6th gear is your crusing gear, ignore it until you need it. If you need more torque, downshift. This higher level of control means that you can compensate for taller gears with your own brain, rather than change the gears.

Overall information.

- 10% of the people who talk in forums are the ones giving out the info, the other 90% are the ones reading and commenting, but not being authorities. The way you become an authority is with experience. Normally these people will have large tires, and new gears, and metal flares, their rigs are crazy. They rock crawl moab, and rubicon trail etc. You know what is better for rock crawling? An automatic transmission. We were on the trail with RK and RIPP and about 50 rigs. There was one manual transmission there. Black Betty.

- Taller gears- people think taller gears have higher numbers, the high numbers are smaller gears. 3.21 is "taller" than 4.10