Choosing the proper gear ratio.
Junior Member
Joined: Dec 2011
Posts: 39
Likes: 0
From: Richmond, VA
Year: 95
Model: Cherokee
Thanks for some of that advice man. So I'm just wondering if anyone can help me understand what exactly to look for in an axle to make sure that it fits into my 97 XJ. Also I know the gears are reversed cut on the XJ, what vehicles could I work at getting gears out of. I'm just buying parts off of wall rigs on Facebook marketplace so I just want to know what to look for if anyone can help me with that. Thank y'all.
Second option is keep your eyes out for people that have already built/modified a set of axle for an XJ, you may be able to find a lock or LS 8.8 with a matching geared front for sale cheap. Just take your time and don't just buy anything that comes up for sale. If you are planning to do it in a few months take the time to get the easier of parts to put under your rig unless you have the welding/fabrication/regearing skills to do the full build yourself.
Thread Starter
CF ADMIN
Joined: Sep 2010
Posts: 34,087
Likes: 256
From: Lantana, Fl
Year: 1996
Model: Cherokee (XJ)
Engine: 4.Slow
Not really when off-road or around town. 4.56:1 would have been ideal. The only downside I see is your RPMs will be higher on the highway. But the positive is if you go to 35's, you won't need to regear. 28 ÷ 35 x 4.88 = 3.90:1
Junior Member
Joined: Nov 2017
Posts: 80
Likes: 1
Year: 1997
Model: Cherokee(XJ)
Engine: 4.0L
Hey guys, this is more of an axle question than a gear ratio question but they still go hand-in-hand (also I cross posted it in order to hopefully get help soon). So right now I have a 97XJ on a front d30 and rear d35 and I'm running 3.55 gears on 30in tires. I just bought myself a new set of 33in tires and I'm about $70 away from getting my 4.5 RC lift ordered. So I have a few questions:
I know I'm goin to have to regear my jeep sooner than later to 4:10, but how important will that be to do right away. Also I do drive the freeway a decent amount with it so I know it will need to be done. But this leads me to my next question. Once I buy the lift kit here in a week or so, the next thing on my list are new axles. Quite frankly i just am looking for advice on what axles I should be searching for and if there are requirement I need to know in order to tell if an axle will be functionable under my jeep, I've been looking mostly at d44s. Another thing I'm wondering is if I were to just replace my rear to say a Dana 44 (hypothetically), would I then need to replace my front to a Dana 44 also or could I keep the D30 and just have matching gears. And lastly, is there any advice I should have on the order I should install these, obviously I can't put the tires on until I have the lift but should I wait for a new axle before I put anything on.
since this group chat is specifically about gear ratio and I don't want to get banned from the chat for talking about my axle and lift questions, maybe I can just focus on how well my jeep will run on 33s w 3.55 gears?
I know I'm goin to have to regear my jeep sooner than later to 4:10, but how important will that be to do right away. Also I do drive the freeway a decent amount with it so I know it will need to be done. But this leads me to my next question. Once I buy the lift kit here in a week or so, the next thing on my list are new axles. Quite frankly i just am looking for advice on what axles I should be searching for and if there are requirement I need to know in order to tell if an axle will be functionable under my jeep, I've been looking mostly at d44s. Another thing I'm wondering is if I were to just replace my rear to say a Dana 44 (hypothetically), would I then need to replace my front to a Dana 44 also or could I keep the D30 and just have matching gears. And lastly, is there any advice I should have on the order I should install these, obviously I can't put the tires on until I have the lift but should I wait for a new axle before I put anything on.
since this group chat is specifically about gear ratio and I don't want to get banned from the chat for talking about my axle and lift questions, maybe I can just focus on how well my jeep will run on 33s w 3.55 gears?
CF Veteran
Joined: Nov 2015
Posts: 3,636
Likes: 469
From: Southern OH
Year: 1997
Model: Cherokee(XJ)
Engine: 4.0
Hey guys, this is more of an axle question than a gear ratio question but they still go hand-in-hand (also I cross posted it in order to hopefully get help soon). So right now I have a 97XJ on a front d30 and rear d35 and I'm running 3.55 gears on 30in tires. I just bought myself a new set of 33in tires and I'm about $70 away from getting my 4.5 RC lift ordered. So I have a few questions:
I know I'm goin to have to regear my jeep sooner than later to 4:10, but how important will that be to do right away. Also I do drive the freeway a decent amount with it so I know it will need to be done. But this leads me to my next question. Once I buy the lift kit here in a week or so, the next thing on my list are new axles. Quite frankly i just am looking for advice on what axles I should be searching for and if there are requirement I need to know in order to tell if an axle will be functionable under my jeep, I've been looking mostly at d44s. Another thing I'm wondering is if I were to just replace my rear to say a Dana 44 (hypothetically), would I then need to replace my front to a Dana 44 also or could I keep the D30 and just have matching gears. And lastly, is there any advice I should have on the order I should install these, obviously I can't put the tires on until I have the lift but should I wait for a new axle before I put anything on.
since this group chat is specifically about gear ratio and I don't want to get banned from the chat for talking about my axle and lift questions, maybe I can just focus on how well my jeep will run on 33s w 3.55 gears?
I know I'm goin to have to regear my jeep sooner than later to 4:10, but how important will that be to do right away. Also I do drive the freeway a decent amount with it so I know it will need to be done. But this leads me to my next question. Once I buy the lift kit here in a week or so, the next thing on my list are new axles. Quite frankly i just am looking for advice on what axles I should be searching for and if there are requirement I need to know in order to tell if an axle will be functionable under my jeep, I've been looking mostly at d44s. Another thing I'm wondering is if I were to just replace my rear to say a Dana 44 (hypothetically), would I then need to replace my front to a Dana 44 also or could I keep the D30 and just have matching gears. And lastly, is there any advice I should have on the order I should install these, obviously I can't put the tires on until I have the lift but should I wait for a new axle before I put anything on.
since this group chat is specifically about gear ratio and I don't want to get banned from the chat for talking about my axle and lift questions, maybe I can just focus on how well my jeep will run on 33s w 3.55 gears?
Member
Joined: May 2016
Posts: 223
Likes: 47
From: Castle Rock, CO
Year: 1987
Model: Comanche (MJ)
Engine: 5.3
Late to the party here. But I have something hopefully useful to add.
Years ago I wrote a little Java applet on my website and a few people have found it useful for calculating your gear ratios and the effect that tire size changes and gear ratio changes work together. I suspect a few of you have seen it before:
http://www.grimmjeeper.com/gears.html
When I was doing the gears in my XJ last year, I also figured out the math for the speedometer gear that you would put in the transfer case. I put in most of the tire sizes I could find and corrected for the fact that most tires are smaller than advertised. I color coded the chart to match the color of the gear when you buy it. I'm running 31x10.50/15 tires on my XJ with 4.27 gears in the Grand Wagoneer Dana44 axles I stuffed under it and used this chart to pick the speedometer gear. My speedometer is just about dead nuts on to my GPS. Same with my J10 running 315/75R15 tires with 4.56 gears.
Years ago I wrote a little Java applet on my website and a few people have found it useful for calculating your gear ratios and the effect that tire size changes and gear ratio changes work together. I suspect a few of you have seen it before:
http://www.grimmjeeper.com/gears.html
When I was doing the gears in my XJ last year, I also figured out the math for the speedometer gear that you would put in the transfer case. I put in most of the tire sizes I could find and corrected for the fact that most tires are smaller than advertised. I color coded the chart to match the color of the gear when you buy it. I'm running 31x10.50/15 tires on my XJ with 4.27 gears in the Grand Wagoneer Dana44 axles I stuffed under it and used this chart to pick the speedometer gear. My speedometer is just about dead nuts on to my GPS. Same with my J10 running 315/75R15 tires with 4.56 gears.
Last edited by derf; Apr 24, 2018 at 04:53 PM.
Thread Starter
CF ADMIN
Joined: Sep 2010
Posts: 34,087
Likes: 256
From: Lantana, Fl
Year: 1996
Model: Cherokee (XJ)
Engine: 4.Slow
Late to the party here. But I have something hopefully useful to add.
Years ago I wrote a little Java applet on my website and a few people have found it useful for calculating your gear ratios and the effect that tire size changes and gear ratio changes work together. I suspect a few of you have seen it before:
http://www.grimmjeeper.com/gears.html
When I was doing the gears in my XJ last year, I also figured out the math for the speedometer gear that you would put in the transfer case. I put in most of the tire sizes I could find and corrected for the fact that most tires are smaller than advertised. I color coded the chart to match the color of the gear when you buy it. I'm running 31x10.50/15 tires on my XJ with 4.27 gears in the Grand Wagoneer Dana44 axles I stuffed under it and used this chart to pick the speedometer gear. My speedometer is just about dead nuts on to my GPS. Same with my J10 running 315/75R15 tires with 4.56 gears.
Years ago I wrote a little Java applet on my website and a few people have found it useful for calculating your gear ratios and the effect that tire size changes and gear ratio changes work together. I suspect a few of you have seen it before:
http://www.grimmjeeper.com/gears.html
When I was doing the gears in my XJ last year, I also figured out the math for the speedometer gear that you would put in the transfer case. I put in most of the tire sizes I could find and corrected for the fact that most tires are smaller than advertised. I color coded the chart to match the color of the gear when you buy it. I'm running 31x10.50/15 tires on my XJ with 4.27 gears in the Grand Wagoneer Dana44 axles I stuffed under it and used this chart to pick the speedometer gear. My speedometer is just about dead nuts on to my GPS. Same with my J10 running 315/75R15 tires with 4.56 gears.
Thread Starter
CF ADMIN
Joined: Sep 2010
Posts: 34,087
Likes: 256
From: Lantana, Fl
Year: 1996
Model: Cherokee (XJ)
Engine: 4.Slow
Seasoned Member
Joined: Oct 2017
Posts: 325
Likes: 15
From: NW OH
Year: 1991
Model: Cherokee (XJ)
Engine: 4.0L L6 OHV 12V
From the Quadratec tire size/gear ratio page on their website...”Please NOTE: If you have an automatic transmission your RPM will higher due to slippage in the transmission and the torque converter. With an overdrive transmission your RPM in overdrive will be 15% to 30% less than indicated.”
This reference above is to the QT tire size/gear ratio chart. I have a 5 speed manual. Does that qualify as an “overdrive transmission?”
I am regearing my Dana’s to 4.10. Looking at the tire size/gear ratio chart... with 31’s, will be running 2,896 RPMs at highway speed. But based on the quote above, will I be turning 15%-30% lower RPMs in 5th gear on the highway?
This reference above is to the QT tire size/gear ratio chart. I have a 5 speed manual. Does that qualify as an “overdrive transmission?”
I am regearing my Dana’s to 4.10. Looking at the tire size/gear ratio chart... with 31’s, will be running 2,896 RPMs at highway speed. But based on the quote above, will I be turning 15%-30% lower RPMs in 5th gear on the highway?
CF Veteran
Joined: Nov 2015
Posts: 3,636
Likes: 469
From: Southern OH
Year: 1997
Model: Cherokee(XJ)
Engine: 4.0
That chart is based on a non overdrive transmission, 5th gear in your 5 speed is a .79 ratio so it's overdrive. Use this calculator to get your real numbers. http://www.grimmjeeper.com/gears.html
Or use the formula,
RPM= MPH x transmission gear ratio x axle gear ratio x 336 divided by tire diameter.
Or use the formula,
RPM= MPH x transmission gear ratio x axle gear ratio x 336 divided by tire diameter.
Seasoned Member
Joined: Oct 2017
Posts: 325
Likes: 15
From: NW OH
Year: 1991
Model: Cherokee (XJ)
Engine: 4.0L L6 OHV 12V
That chart is based on a non overdrive transmission, 5th gear in your 5 speed is a .79 ratio so it's overdrive. Use this calculator to get your real numbers. http://www.grimmjeeper.com/gears.html
Or use the formula,
RPM= MPH x transmission gear ratio x axle gear ratio x 336 divided by tire diameter.
Or use the formula,
RPM= MPH x transmission gear ratio x axle gear ratio x 336 divided by tire diameter.
The 336 is interesting...
Last edited by Rlmx; Feb 28, 2019 at 06:49 AM.