Larger tires, gearbox gear upgrades??

[Cherokee85]

Hi guys

just a quick one for you, I stumbled across a website I cant find now, trying to find it again, basically when you fit larger tires to your jeep you lose some performance and power, correct?

well I found a website that you can buy gearbox parts to make it drive like it has normal tires on it,

It all may sound odd to you guys, but does anyone know what I'm on about and trying to aim at? LOL

thanks guys

Joey

[NW99XJ]

Hi guys

just a quick one for you, I stumbled across a website I cant find now, trying to find it again, basically when you fit larger tires to your jeep you lose some performance and power, correct?

well I found a website that you can buy gearbox parts to make it drive like it has normal tires on it,

It all may sound odd to you guys, but does anyone know what I'm on about and trying to aim at? LOL

thanks guys

Joey
Hi Joey, this is a common question/concern that pops up frequently,....
But in a nutshell, you're on the right train of thought.
What happens is the larger tires you put on, change the gear ratio of the drive train....so for every ONE revolution of the transmission's output you are effectively reducing the number of rotations of the tire as you increas tire size.

In our case we dont change gears in a gear box...we change the gears in our differentials.... (yes BOTH at the same time - or you'll cause some serious damage if you put it into 4wd)

To determine what gears you need/want we'll refer to the charts available online. (see below)
As a reference.... most 6cyl cherokees came with either 3.73 or more commonly 3.55 gear ratios.....
Some 4cyl cherokees came with 4.10
As a rule of thumb or most commonly practiced.....here's a breakdown of what most people choose when it comes to tires and gear sizes respectively:
(NOTE: Gear sizes listed below are the most commonly available)
30-31" tires - 4.10 gears
32-33" tires - 4.56 gears
35-37" tires - 4.88 gears
38-40" tires - 5.13 gears

For a more accurate look at how this works, and to give you a better tool to determine your OWN gear ratio...see this chart:
http://i41.tinypic.com/30a6dr8.jpg

[nickyg]

I never heard of any gear box parts. I have heard of swapping out the whole gearbox with a beefier one.
I have that upgrade on my build list.
The gear box out of certin years durango fit the XJ and are priced very reasonable. Thats just my .02 I'm sure we'll hear from someone who has done it before and will tell us the pros/ cons.

[nickyg]

See, in the time it took me to type that (I use the hunt and peck method), Josh came to the rescue.
Thanks, you answered some questions I had too.

[nickyg]

And now that I reread the question, I misunderstood it totally , Sorry... Just ignore me.. I need more coffee.

[Cherokee85]

thank you NW99XJ ! :) massive help

its just that when ive got my XJ Sport 2.5L lifted I would like bigger tires but those questions went into my head "will lose power how will I get decent power back?" "will it damage gearbox?"

now I am already going to upgrade the gearbox from the AX5 to the AX15 found on the 4.0L Manuanls and over here in Europe its found on the 2.5 VM diesel Cherokees.

so it's more the diff I need to sort out, nice one, thank you so much man! :)


and no worries nickyg :) coffee is good!


Joey

[4.3LXJ]

There is one modification that is helpful when you change gears or tires. The speedometer will be off, so you can change the speedometer drive gear in the tail housing. It is easy and a nice upgrade when you change those things. They come in many different tooth counts and are easy to change

[Cherokee85]

hey 4.3LXJ

yeah I found those on the internet, cheap and easy to fit so they say :) and I have found the website I looked at for the gain performance back issue with larger tires

http://www.jeep4x4center.com/jeep-cherokee/performance-index.htm

and I think quadratec also do these parts as well.

Thank you guys! Want my XJ to be useable with many toys :)

Joey

[4.3LXJ]

Lots of sites have gears and related parts. If you regear are you going to lock it while you are at it?

[Cherokee85]

Possible yes. I'm just thinking when I have the funds for the work I'm gonna do it all in one go :)

[Carves]

Gearbox ratios are still something to consider when your working out tyre sizes and diff ratios.

Charts like the one posted above generally base the figures on a 1:1 gearbox ratio ... and if you spend a lot of time in 5th/Overdrive then the rev count will be lower than the chart.

Im about to drop 4.10 diffs in my AW4 auto 4.0l - and even tho 3.73 would probably been better with 30" tyres, in regards to long term fuel efficiency ... I figure that with 4.10 ..maybe I'll use 4th gear a lot more often in the future .. :D


TRANSMISSION GEAR RATIOS

AX5 manual transmission:
First gear: 3.93:1
Second gear: 2.33:1
Third gear: 1.45:1
Fourth gear: 1.00:1
Fifth gear: 0.85:1
Reverse gear: 4.74:1

AX15 manual transmission:
First gear: 3.83:1
Second gear: 2.33:1
Third gear: 1.44:1
Fourth gear: 1.00:1
Fifth gear: 0.79:1
Reverse: 4.22:1

AW4 auto transmission:
First gear: 2.804:1
Second gear: 1.531:1
Third gear: 1.000:1
Fourh gear: 0.753:1
Reverse: 2.393:1

[Paddletrucker]

Hey guys, I notice the original poster, Cherokee85, has a manual tranny. How does a manual tranny change the need for different ratios? I know I can pretty much idle along in 4th gear as long as I don't climb a hill. I've thought about just finding a stock Cherokee set of axles with 3.55's once I go to the 31" tires. 3.73's would be cool, too. I love my 5 speed, but do wish I had a granny gear though.:D making it a 6 speed. That'd be awesome for my farm working Jeep!!:thumbsup:

What's the difference in an AX5 and an AX15?? I thought I had the AX15. AAm I wrong??

[gary63]

I think he was thinking of gear vender they have a gear box that bolts up to your tran. and double your gears but it only works in 2wd not 4wd.

[4.3LXJ]

Hey guys, I notice the original poster, Cherokee85, has a manual tranny. How does a manual tranny change the need for different ratios? I know I can pretty much idle along in 4th gear as long as I don't climb a hill. I've thought about just finding a stock Cherokee set of axles with 3.55's once I go to the 31" tires. 3.73's would be cool, too. I love my 5 speed, but do wish I had a granny gear though.:D making it a 6 speed. That'd be awesome for my farm working Jeep!!:thumbsup:

What's the difference in an AX5 and an AX15?? I thought I had the AX15. AAm I wrong??

The real difference is in final gear ratio in overdrive, or 5th to you truckers. :P

The final gear ratio is different. You have a 14% overdrive while the auto has a 25%. So when they made your Jeep, instead of 3.54 differential gear ratio it has a 3.07. So if you go to 4.10s, this would allow you to go to 33s more comfortably than with an auto. For what you use yours for, 410s would be good with 31s.

The AX5 was used behind the smaller engines, which also had 4.10s in them from the factory. So if you want to convert cheap, get some axles out of the XJs with the 2.5 or 2.8 engine in it and it will swap right in and be cheaper to boot