DODGE RAM FORUM banner

1 - 15 of 15 Posts

·
Registered
Joined
·
20 Posts
Discussion Starter #1
Is it necessary to reprogram the speedometer after changing differential gears from 3.92 to 3.21. I just completed the swap and it seems my speedometer is correct. I don't have an easy way to judge, but traffic flow is steady. Also, with the 3.92 gears at 70mph I would tach almost 2,000rpm. After putting in 3.21's I tach 1500rpms at 70mph. Thanks for your help.:4-dontknow:
 

·
Super Moderator
Joined
·
34,714 Posts
You went FROM 3.92s TO 3.21s?!?!?!

Bubba, get in here!!! We found one!!!!:LOL:

I'm not making fun of you OP, just most guys want to go from 3.21s to 3.92s. You have the 8 speed right? Would you mind explaining your reasoning?

I have 3.21s too FWIW. I'm asking because people are always talking trash about 3.21s.

Mathematically, 3.21s put you at 68.44 MPH at exactly 1500 RPM. 3.92s do 56.04 MPH at 1500 RPM, so you should be able to notice if it were off, but IDK how the spedometer works. It could be based on transmission output shaft speed or wheel speed, and you may or may not need a calibration based on which it is. Source: http://www.ramforumz.com/showthread.php?t=142580

It sounds to me like its based off wheel speed because of what you are saying but I'm not sure.

Time to stick it to Bunkshuss!:box2::LOL:
 

·
Registered
Joined
·
35 Posts
Holy Mackrel..... 3.92 to 3.21's????
Are you crazy man? What is everybody else gonna say???
There is nothing better on earth than 3.92's, don't you know that? ;)

Sorry.... I can't help. Just had to get that out of my system. I feel better now.
 

·
Registered
Joined
·
20 Posts
Discussion Starter #4
The truck is still a rocket with the 3.21 gears. It is an attempt to improve gas mileage. I spent $350 for set of gears which included a posi-traction carrier and all the bearings. My 3.92 was not posi-traction My local garage charged me $254 to install and I spent $50 on gear lube (Mobil 1) synthetic. So I have $650 in the swap-out. I can probably sell the 3.92 gears on Ebay for $300 to $400 so final cost for swap-out about $300.
 

·
Registered
Joined
·
3,492 Posts
You went FROM 3.92s TO 3.21s?!?!?!

Bubba, get in here!!! We found one!!!!:LOL:

I'm not making fun of you OP, just most guys want to go from 3.21s to 3.92s. You have the 8 speed right? Would you mind explaining your reasoning?

I have 3.21s too FWIW. I'm asking because people are always talking trash about 3.21s.

Mathematically, 3.21s put you at 68.44 MPH at exactly 1500 RPM. 3.92s do 56.04 MPH at 1500 RPM, so you should be able to notice if it were off, but IDK how the spedometer works. It could be based on transmission output shaft speed or wheel speed, and you may or may not need a calibration based on which it is. Source: http://www.ramforumz.com/showthread.php?t=142580

It sounds to me like its based off wheel speed because of what you are saying but I'm not sure.

Time to stick it to Bunkshuss!:box2::LOL:
I'm here this post woke me up. Doesn't he know that with 3.21 gears it can't haul anything in the bed no more and it won't get out of it's on way. And don't forget the sick to your stomach feeling when you see a big bad sporty with the fake ram air HOOD with the 3.92 in your mirror. Nothing is worse then that. Lmao:LOL::LOL:
 

·
Resident Technician
Joined
·
14,202 Posts
Some would argue that economy will suffer because the engine now has to work harder to get the truck up to highway speed. I suppose if you live in a flat area, this isn't a big deal, but in the mountains of East Tennessee, 3.21 gears are a serious liability to anyone wanting to tow or haul on a regular basis.
 

·
Registered
Joined
·
143 Posts
4x4?? Did you switch the front out too? I'd like to go from 3.21 to 3.92 but it's a pretty expensive job for parts/labour up here in Canada that I can't justify just yet..
 

·
Registered
Joined
·
3,492 Posts
With the 8 speed it makes a huge difference. If you don't have one you really don't know. It doesn't compare to the old trucks.
 

·
Moderator
Joined
·
8,070 Posts
To answer the OP's question... no, you don't have to reprogram the speedo for a gear swap on these trucks. The speedometer sensor is in the rear differential and reads off a ring on the carrier. The carrier is the same size regardless of the ring and pinion size so the speedo is correct with either gear ratio. The only time a speedometer recalibration is required is when the overall tire size is changed.
 

·
Registered
Joined
·
29 Posts
I went from 3.21 to 3.92 gears and my gas mileage is slightly higher with the lower gears, but I do mostly small stop and go driving. The opposite will be true though if you do mostly hwy driving.
 

·
Registered
Joined
·
96 Posts
I am looking to do the exact same thing. I bought the 3.92 because the truck had everything I wanted. I drive mostly highway and don’t need the extra rear gear.

What was the part number of the rear gear? Where did you get it?

I live in South Florida, if you are local, what shop did the swap?

Thanks in advance.
 

·
Registered
Joined
·
20 Posts
Discussion Starter #12
I got gears off of Ebay. They were $350 including shipping and that included a posi-traction carrier, and all bearings. I am in Clermont outside Orlando. The dealer wanted $800 plus parts to do swap out. Plus $780 for gears.
 
1 - 15 of 15 Posts
Top