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Old 05-22-2018, 10:52 PM   #1
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How does one PROVE the axle ratio of a used truck?

Hey all. Currently, we have a little 22 foot TT and looking to upgrade our TV with an eye towards our *next* camper (10,000 or less, fully loaded) We have our eye on a 2012 Ram 2500 5.7, crew cab, 4x4 LB The dealer says it has a 3:73 axle however according to the VIN at this site below, it has a 4:10 This COULD represent up to a 3000# difference in max tow ratings, according to the 2012 Trailer Life Towing Guide

https://www.dodge.com/webselfservice/dodge/index.html


One question I have is, which to believe? (the VIN!?) but also...if you had absolutely NO idea, how would one determine exactly which rear diff he actually has?
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Old 05-22-2018, 10:57 PM   #2
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Originally Posted by gorgerax View Post
Hey all. Currently, we have a little 22 foot TT and looking to upgrade our TV with an eye towards our *next* camper (10,000 or less, fully loaded) We have our eye on a 2012 Ram 2500 5.7, crew cab, 4x4 LB The dealer says it has a 3:73 axle however according to the VIN at this site it has a 4:10 This COULD represent up to a 3000# difference in max tow ratings, according to the 2012 Trailer Life Towing Guide

https://www.dodge.com/webselfservice/dodge/index.html
file:///C:/Users/gorge/Downloads/2012_towing_guide_lo_res.pdf

One question I have is, which to believe? (the VIN!?) but also...if you had absolutely NO idea, how would one determine exactly which rear diff he actually has?

There should be a tag on the axle that says the ratio, or gives you a BOM number (bill of materials) that you can look up.
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Old 05-22-2018, 11:02 PM   #3
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That's a good question, I had my truck custom built and only the build sheet and the window sticker are correct. All the web services return false information in one way or another.
Having said this, the max tow rating is not a number that's written in stone or legally binding AFAIK, but shouldn't be ignored either.
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Old 05-22-2018, 11:08 PM   #4
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The Dodge URL you used pulls the equipment listing as the truck was built. Believe that over what anyone else tells you.
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Old 05-22-2018, 11:16 PM   #5
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How does one PROVE the axle ratio of a used truck?

What you could do is jack the rear end up on the truck, make a mark on the drive shaft and a mark on one rear tire.
On the tire, rotate the tire 1 full revolution and count how many times the drive shaft turns past the mark you made on the shaft, this is your axel ratio.
For example. 1 full rotation of one rear tire equals 3.73 turns of the drive shaft.

You may have to put the auto trans in neutral when you do this so the drive shaft turns.
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Old 05-23-2018, 04:39 AM   #6
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What was just stated by cfiowa is the old school but as long as you don’t get distracted very reliable
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Old 05-23-2018, 05:17 AM   #7
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What was just stated by cfiowa is the old school but as long as you don’t get distracted very reliable
Old school or not he is correct,It's "Simple Math"! But since it is a "Used Truck" if they have also Changed the "Tire Size" all bets are off in regards to "Tow Rating"! Youroo!!
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Old 05-23-2018, 05:18 AM   #8
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What you could do is jack the rear end up on the truck, make a mark on the drive shaft and a mark on one rear tire.
On the tire, rotate the tire 1 full revolution and count how many times the drive shaft turns past the mark you made on the shaft, this is your axel ratio.
For example. 1 full rotation of one rear tire equals 3.73 turns of the drive shaft.

You may have to put the auto trans in neutral when you do this so the drive shaft turns.
If you are unsure whether you have a Posi Rear or not

Be sure to use jack stands under the axle for safety precaution***
Jack both wheels off the ground and turn one wheel.
•If the opposite wheel turns the same direction or doesn't turn at all, then the rear axle is posi or limited slip.
•If the opposite wheel turns the opposite direction, then it's probably an open rear axle (non-posi).

Determining a Rear Axle Ratio of a Non-Posi Rear
1.Jack one wheel off the ground, put the transmission in neutral and take the parking break off. ***Be sure to use jack stands under the axle for safety precaution***
2.Mark the wheel and the drive shaft with chalk or masking tape.
3.Turn the wheel TWO revolutions and count the number of driveshaft turns.
4.The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and a half times, it's a 3.50:1 ratio. If it turned 4 and a quarter times, it's a 4.25:1 ratio.
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Old 05-23-2018, 05:28 AM   #9
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What was just stated by cfiowa is the old school but as long as you don’t get distracted very reliable
Only correct if it is a limited slip/posi rear end. Not correct if it is an Open rear end.
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Old 05-23-2018, 05:43 AM   #10
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Originally Posted by cfiowa View Post
What you could do is jack the rear end up on the truck, make a mark on the drive shaft and a mark on one rear tire.
On the tire, rotate the tire 1 full revolution and count how many times the drive shaft turns past the mark you made on the shaft, this is your axel ratio.
For example. 1 full rotation of one rear tire equals 3.73 turns of the drive shaft.

You may have to put the auto trans in neutral when you do this so the drive shaft turns.
What he said.
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Old 05-23-2018, 06:37 AM   #11
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When I called it old school no disrespect intended I believe that if not broke don’t fix in most situations
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Old 05-23-2018, 09:07 AM   #12
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Call a dealer with the vin#. Auto Zone should be able to do the same. Unless it was switched.
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Old 05-23-2018, 09:13 AM   #13
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Originally Posted by JCDMeyer View Post
Call a dealer with the vin#. Auto Zone should be able to do the same. Unless it was switched.


No need, the OP already stayed they used Dodge’s website to pull the equipment list (build sheet), the problem is that info doesn’t jive with what the dealer says.

Unless there’s a reason to believe the truck was regeared, trust the manufacturer’s build sheet.
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Old 05-23-2018, 01:11 PM   #14
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On a used truck, tires could be different and the PO might have changed out the rear end for some reason. The definitive, albeit "old school" method of determining the ratio is to count the teeth on the ring gear and divide it by the number of the teeth on the pinion gear.
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Old 05-23-2018, 01:30 PM   #15
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Determining a Rear Axle Ratio of a Non-Posi Rear
1.Jack one wheel off the ground, put the transmission in neutral and take the parking break off. ***Be sure to use jack stands under the axle for safety precaution***
2.Mark the wheel and the drive shaft with chalk or masking tape.
3.Turn the wheel TWO revolutions and count the number of driveshaft turns.
Why do you say to turn the tires TWO revolutions? One revolution should do it!
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Old 05-23-2018, 02:11 PM   #16
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Probably because 2 turns give less chance for error.
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Old 05-23-2018, 02:15 PM   #17
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Why do you say to turn the tires TWO revolutions? One revolution should do it!
Because of the differential in an open rear end. Google it!
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Old 05-23-2018, 02:25 PM   #18
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I didn’t notice that it was determined to be a standard differential. I don’t need to google it after 50 plus years in the truck repair business, I have built many differentials, added non-slip, Detroit lockers and so on. Still do a few in my home shop.
One could also mark both tires and have a second person help turn so they both turn equally, and count the driveshaft revolutions.
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Old 05-23-2018, 02:30 PM   #19
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I didn’t notice that it was determined to be a standard differential....One could also mark both tires and have a second person help turn so they both turn equally, and count the driveshaft revolutions.
You are correct sir.
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Old 05-23-2018, 02:36 PM   #20
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I am not 100% sure on Dodge products. But on GM vehicles there is a list of all the equipment the truck has in the glove box. There will be many codes. Just Google factory rear axle codes for your year, make, and model. Then match up the code.
GM axle codes are GU and a number. Hope this helps.
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