How do I determine my axle gear ratio?

Alex01 said:
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.
Click to expand...

Wow, says mine came with a leather wrapped steering wheel. Didn't know that. I've had a cover on it for so long I'm not sure if that's correct.

Edit: What do ya know? It was right. Didn't even notice.
 
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Thanks for this write-up. Jeep newb here. Picked up a 2000 TJ 4.0 auto transmission. Took it up the hills recently and found it rather gutless. Used your guide here and found out why! It's got 33" and found the diff cover showed 3.07. If I want to tow a little utility trailer while hunting would I need 4.88 gear ratio based on the chart?
 
Elthryn said:
Thanks for this write-up. Jeep newb here. Picked up a 2000 TJ 4.0 auto transmission. Took it up the hills recently and found it rather gutless. Used your guide here and found out why! It's got 33" and found the diff cover showed 3.07. If I want to tow a little utility trailer while hunting would I need 4.88 gear ratio based on the chart?
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I'd bet @Jerry Bransford will tell you 4.56. I'd recommend you go with whatever his recommendation is.

I have 4.88 in my 5 speed with 35s. While it is perfect for the street I wish I would have gone with 5.13 or 5.38 for offroading.
 
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...
Thanks, I have found the tag and it says it a 3.73 ,
 
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...
The link for the build sheet is broken.....
 
Chris said:
Oh yeah, if you have a 1997 there’s nothing you can do unfortunately.
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I have been able to pull many pre-98 vins with no issues whatsoever despite that disclaimer always being there. Worth a try. It doesn't always work but has much in the past.
 
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I have an app on my phone called Speedometer. measures speed and distance. I have 33's on my TJ and when the speedometer on the Jeep shows anything below about 40 it is accurate, but above that it progressively shows to be slower than the phone app, until 50 is really 55 and so on....????
 
BillW said:
I have an app on my phone called Speedometer. measures speed and distance. I have 33's on my TJ and when the speedometer on the Jeep shows anything below about 40 it is accurate, but above that it progressively shows to be slower than the phone app, until 50 is really 55 and so on....????
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That makes no sense unless the speedo gauge is bad or you aren't reading the needle exactly right for 40 and below.
 
BillW said:
Well, I was trying to drive at the same time....Not smart, I know....
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I'd keep a closer eye on it. If it's off by 5 at 50, it should be off proportionally, so 4 at 40, 3 at 30, etc. If it is proportionally off like it is supposed to be with large tires, then all you need to do is a speedo gear change.
 
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