How do I determine my axle gear ratio?

Chris

Chris

Administrator
Staff Member
Ride of the Month Winner
Joined
Sep 28, 2015
Messages
71,097
Location
Gillette, WY
There are a few ways to determine your axle gear ratio:

1. Look for a tag under one of the diff cover bolts in the 9 o'clock position. This will tell you what ratio the Jeep originally had and likely should still have. If someone would have regeared it, chances are they would not put the tag back on. Here is an example of an axle tag you would find on a TJ axle:

Dana axle tag.jpg



2. Get your build sheet / equipment listing instantly from Jeep by clicking here or request a build sheet from a Jeep dealer. This will tell you the ratio the Jeep should have left the factory with. Build sheets can be wrong and axles can be regeared or swapped out, so this does not guarantee 100% accuracy.

3. Jack up the rear end. Count driveshaft revolutions while rotating the tire. Turn the tire 2 complete rotations if your other tire remains stationary and does not rotate. Turn the tire 1 complete rotation if the other tire turns in the same direction. The number of times the driveshaft rotates equals the gear ratio. If the driveshaft rotates about 3x, the gear ratio is 3.07. If it rotates 3 3/4 turns, the gear ratio is 3.73. You can mark the driveshaft to help you count. NOTE: see post below if interested why you spin a tire 1 rotation or 2 rotations.

4. Log your RPM at 60 MPH in a 1:1 transmission gear and plug the numbers in the formula below:

RPM x tire diameter​
—————————- = Gear ratio​
60 x 336​

5. Open the diff cover and count the number of teeth on the ring and pinion gears. The number of teeth on the ring gear divided by the number of teeth on the pinion gear equals the gear ratio. The tooth counts will also often be stamped in to the side of the ring gear. For example, a 3.73 gear set is pictured below (41 divided by 11).

Gear ratio stamped on ring.jpg
 
  • Like
Reactions: Mason Schmidt
Thanks , I'm under the assumption that stock 4:10 but I'll check and see if the tag is there.

I'm just getting lower RPMs and I expected.
 
Chris said:
There are a few ways to determine your axle gear ratio:

1. Look for a tag under one of the diff cover bolts in the 9 o'clock position. This will tell you what ratio the Jeep originally had and likely should still have. If someone would have regeared it, chances are they would not put the tag back on. Here is an example of an axle tag you would find on a TJ axle:

View attachment 99


2. Get your build sheet/equipment listing instantly from Jeep by clicking HERE or request a build sheet from a Jeep dealer. This will tell you the ratio the Jeep should have left the factory with. Build sheets can be wrong and axles can be regeared or swapped out, so this does not guarantee 100% accuracy.

3. Jack up the rear end. Count driveshaft revolutions while rotating the tire. Turn the tire 2 complete rotations if your other tire remains stationary and does not rotate. Turn the tire 1 complete rotation if the other tire turns in the same direction. The number of times the driveshaft rotates equals the gear ratio. If the driveshaft rotates about 3x, the gear ratio is 3.07. If it rotates 3 3/4 turns, the gear ratio is 3.73. You can mark the driveshaft to help you count. NOTE: see post below if interested why you spin a tire 1 rotation or 2 rotations.

4. Log your RPM at 60 MPH in a 1:1 transmission gear and plug the numbers in the formula below:

RPM x tire diameter
—————————- = Gear ratio
60 x 336​

5. Open the diff cover and count the number of teeth on the ring and pinion gears. The number of teeth on the ring gear divided by the number of teeth on the pinion gear equals the gear ratio. The tooth counts will also often be stamped in to the side of the ring gear. For example, a 3.73 gear set is pictured below (41 divided by 11).

View attachment 100
Click to expand...

You state to see post below regarding why some axles have one tire spin and some have both. There is no post explaining that. Could you please explain?
 
actually, if in gear, both axles off the ground, spin one, the other spins backwards
with a locker, locked, in gear nothing moves.
in neutral, spin one tire, either the opposite tire spins or the driveshaft or both.
a limited slip needs enough speed difference to start locking up, on a hoist, it behaves like an open dif.
on the ground, under power, limited slips, work great when the speed differences are high. ie the Hemi Cuda, but crawling, a limited slip might not make any difference until something actually loses traction.

Lockers or spools on the other hand will never let one tire spin. In a straight line a locker wins all contests.

a Detroit locker is a street racers dream. ready to offer instant traction with zero thought control. But while it takes zero thought, it engages with violence, and only under power. On a steep downhill with intermittent traction, loose gravel, boulders, a detroit locker offers nothing. Only a locker.... or 2 make a difference when the power is off. I like my lockers climbing steep loose rough embankments, but I really smile when I go down that embankment with zero drama.
 
Last edited:
billiebob said:
actually, if in gear, both axles off the ground, spin one, the other spins backwards
with a locker, locked, in gear nothing moves.
in neutral, spin one tire, either the opposite tire spins or the driveshaft or both.
a limited slip needs enough speed difference to start locking up, on a hoist, it behaves like an open dif.
on the ground, under power, limited slips, work great when the speed differences are high. ie the Hemi Cuda, but crawling, a limited slip might not make any difference until something actually loses traction.

Lockers or spools on the other hand will never let one tire spin. In a straight line a locker wins all contests.

a Detroit locker is a street racers dream. ready to offer instant traction with zero thought control. But while it takes zero thought, it engages with violence, and only under power. On a steep downhill with intermittent traction, loose gravel, boulders, a detroit locker offers nothing. Only a locker.... or 2 make a difference when the power is off. I like my lockers climbing steep loose rough embankments, but I really smile when I go down that embankment with zero drama.
Click to expand...

What about a Powertrax No Slip?
 
billiebob said:
tape a string to the driveshaft and you don't have to watch the driveshaft while you rotate it.
Click to expand...
Another way to do it is to take a long piece of tape (masking, electrical, etc.). About 16" is plenty. Loosely place it around the shaft, press the ends together with your fingers and tighten the tape up to the shaft. You'll end up with a long tab that is stuck to the shaft and will rotate with it.
 
Chris said:
There are a few ways to determine your axle gear ratio:

1. Look for a tag under one of the diff cover bolts in the 9 o'clock position. This will tell you what ratio the Jeep originally had and likely should still have. If someone would have regeared it, chances are they would not put the tag back on. Here is an example of an axle tag you would find on a TJ axle:

View attachment 99


2. Get your build sheet/equipment listing instantly from Jeep by clicking HERE or request a build sheet from a Jeep dealer. This will tell you the ratio the Jeep should have left the factory with. Build sheets can be wrong and axles can be regeared or swapped out, so this does not guarantee 100% accuracy.

3. Jack up the rear end. Count driveshaft revolutions while rotating the tire. Turn the tire 2 complete rotations if your other tire remains stationary and does not rotate. Turn the tire 1 complete rotation if the other tire turns in the same direction. The number of times the driveshaft rotates equals the gear ratio. If the driveshaft rotates about 3x, the gear ratio is 3.07. If it rotates 3 3/4 turns, the gear ratio is 3.73. You can mark the driveshaft to help you count. NOTE: see post below if interested why you spin a tire 1 rotation or 2 rotations.

4. Log your RPM at 60 MPH in a 1:1 transmission gear and plug the numbers in the formula below:

RPM x tire diameter​
—————————- = Gear ratio​
60 x 336​

5. Open the diff cover and count the number of teeth on the ring and pinion gears. The number of teeth on the ring gear divided by the number of teeth on the pinion gear equals the gear ratio. The tooth counts will also often be stamped in to the side of the ring gear. For example, a 3.73 gear set is pictured below (41 divided by 11).

View attachment 100
Click to expand...
Chris,
In #4 above, do I use the RPM when the tachometer reads 60mph or when the GPS says I am actually doing 60mph?
If I do the GPS way, I am at 2300 rpm and I would calculate to a 3.73 gear ration running 33" tires. (3.76 but I rounded down) Is that correct? (33 x 2300)/(60x336)=3.76 or 75,900/20,160=3.76
If I do the tach reading it doesn't match a gear ratio (33x2000)/(60x336)=3.27

I have a 31 tooth gear following the charts and installed it so the housing rotates to the 6o'clock position for 26-31 per the owners manual.
When I drive I am 10mph too high on the tach.

Does this all make sence? Trying to get the right setup so the tach reads close to accurate so I don't get tickets for going too slow.....LOL
 
I think you mean speedometer not tach :)
31 is the right tooth count for 3.73/31" combo, Here is the chart from Quadratech. If you have it installed in the transmission housing correctly it should be within 1 or 2mph against the GPS. You are probably really dogging it with 3.73 gears. 4.88 or lower would be better. I have 33" with 4.88 and it's still a dog going up hills with the 4 speed auto trans. I need to manual shift out of OD with the button on the console. That is just par for the course with the 4 speed trans.

https://www.quadratec.com/c/reference/jeep-speedometer-gear-tooth-chart
78584
 
Russell Balzan said:
Chris,
In #4 above, do I use the RPM when the tachometer reads 60mph or when the GPS says I am actually doing 60mph?
If I do the GPS way, I am at 2300 rpm and I would calculate to a 3.73 gear ration running 33" tires. (3.76 but I rounded down) Is that correct? (33 x 2300)/(60x336)=3.76 or 75,900/20,160=3.76
If I do the tach reading it doesn't match a gear ratio (33x2000)/(60x336)=3.27

I have a 31 tooth gear following the charts and installed it so the housing rotates to the 6o'clock position for 26-31 per the owners manual.
When I drive I am 10mph too high on the tach.

Does this all make sence? Trying to get the right setup so the tach reads close to accurate so I don't get tickets for going too slow.....LOL
Click to expand...

If you have no reason to believe it has been re geared here is a great place to start https://fcacommunity.force.com/RAM/s/equipment-listing

Type in your vin and you'll know exactly how it left the factory.
 
Thanks Chris (yes I meant speedometer). I also checked the VIN against the build and it said it was 3.73.
Since the case was replaced I assumed it was still geared the same. So I installed the 31 tooth speedometer gear as recommended. It is completely seated and I spun the housing to have 26-31 in the 6 o'clock position as per the owners manual. This is a 3 speed automatic so I don't have OD.
Only other thing I can think of is to replace the electronic sensor that is attached to the gear.
 
I don't know if you can/should do this but maybe bump it to the next spot where it is seated in the housing. Maybe since 31 is right on the edge??
 
Thank you for this! I just purchased my Wrangler and I haven't even picked it up yet, but this was a question my dealer couldn't answer today. (I hope I'm replying correctly 🤣) I don't want you moving me in the morning.
 
  • Like
Reactions: Chris
Hello,

I have a wrangler TJ 2004, 2.4. My stock axle gear ratio is 4.11.

I want to upgrade to 30''-31'' tires (with 2-2.5'' lift).

I have found some charts about the axle gear ratios (i.e. https://www.quadratec.com/c/reference/gear-ratio-guide-for-larger-tires )

I am a little bit confused, because my stock tire size was (215/75r15, which is 27.7''). The chart shows that the best overall performance is with tires 30''-33''.

Am I missing something?
 
Top Bottom
Back