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  #1  
Old 05-18-2013, 03:46 AM
mdragon9 mdragon9 is offline
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Default Comparison of RPM and MPH of 8 speed/3.21 vs 6 speed/3.55

The attached PDF has a table of the calculated speed of a Ram with a 8 speed and 3.21 rear diff vs a 6 speed and 3.55 rear diff. It assumes a 33" tire size.

The 8 speed and 3.21 diff looks to be a great overall combination. I wish it had been available when I bought my truck. Low end is lower and high end is higher. 7th gear in the 8 speed/3.21 is pretty close to 5th gear in the 6 speed/3.55 combo.

I'll be very interested to hear if peoples impressions line up with the calculations. A hemi with the 8 speed and 3.21 should feel snappier overall than a hemi with the 6 speed and 3.55.

To read the chart, along the top is RPM and down the left are the different gear ratios for the two transmissions. The left hand group shows the tire rotations/minute at a particular engine RPM and the right hand group shows MPH at a particular engine RPM.

For example, at 1500 RPM, you should be going about 68 MPH with the 8 speed/3.21 and about 61 MPH with the 6 speed/3.55 combo.

Edit: I've updated the chart to include all combos of rear and transmission. Interesting, the 8 speed with the 3.21 has a lower low than the 6 speed with the 3.92 and 7th gear is roughly equivalent to 6th in the 6 speed.
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File Type: pdf drive ratios - v2.pdf (211.4 KB, 188 views)

Last edited by mdragon9; 05-18-2013 at 11:47 AM. Reason: Updated attachment
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Old 05-18-2013, 08:17 AM
lowfast lowfast is offline
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Great info, anyway to add the 3.92's as well?
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Old 05-18-2013, 08:47 AM
Asur Asur is offline
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Great info, as much as the 8 speed with 3.21 is nice, I find my 6 speed with 3.55 is not bad for a 1/2 ton truck.
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Old 05-18-2013, 09:07 AM
Monster5601 Monster5601 is offline
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Quote:
Originally Posted by lowfast View Post
Great info, anyway to add the 3.92's as well?
Here is a copy of a spreadsheet I did a while back when I first discovered my 3.21 versus 3.55 axle ratio SNAFU.

Oops, guess I can't attached a spreadsheet so here is a screen shot of the output for all three axle ratios.

You can find the formula here:
http://www.ramforumz.com/showpost.ph...&postcount=163
Attached Images
File Type: jpg Ratios.JPG (52.7 KB, 51 views)

Last edited by Monster5601; 05-18-2013 at 09:39 AM.
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Old 05-18-2013, 09:27 AM
Ddaaryl Ddaaryl is offline
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I have had both rear end ratios and see no difference in mileage. The window sticker rates them both the same as well
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Old 05-18-2013, 09:44 AM
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So from my quick math it looks like 3.55's with a 33" tire will turn roughly the same highway RPM's as 3.92's with a 35" tire. Thanks for the info, I was on the fence on going with 3.55's or 3.92's. I have driven both and with the stock tires the truck feels "over geared" to me with the 3.92's, the 3.55's with the 8 spd seem like the perfect combo. Since I plan to level it and run a 295/60/20 tire it looks like the 3.92's would be a good choice.
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Old 05-18-2013, 10:30 AM
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Quote:
Originally Posted by mdragon9 View Post
The attached PDF has a table of the calculated speed of a Ram with a 8 speed and 3.21 rear diff vs a 6 speed and 3.55 rear diff. It assumes a 33" tire size.

The 8 speed and 3.21 diff looks to be a great overall combination. I wish it had been available when I bought my truck. Low end is lower and high end is higher. 7th gear in the 8 speed/3.21 is pretty close to 5th gear in the 6 speed/3.55 combo.

I'll be very interested to hear if peoples impressions line up with the calculations. A hemi with the 8 speed and 3.21 should feel snappier overall than a hemi with the 6 speed and 3.55.

To read the chart, along the top is RPM and down the left are the different gear ratios for the two transmissions. The left hand group shows the tire rotations/minute at a particular engine RPM and the right hand group shows MPH at a particular engine RPM.

For example, at 1500 RPM, you should be going about 68 MPH with the 8 speed/3.21 and about 61 MPH with the 6 speed/3.55 combo.

I love my 8 speed with 3.21, it's very snappy. Feels like a bucking bull when you punch it when trying to pass someone
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Old 05-18-2013, 11:56 AM
mdragon9 mdragon9 is offline
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Quote:
Originally Posted by mdragon9 View Post
The attached PDF has a table of the calculated speed of a Ram with a 8 speed and 3.21 rear diff vs a 6 speed and 3.55 rear diff. It assumes a 33" tire size.

The 8 speed and 3.21 diff looks to be a great overall combination. I wish it had been available when I bought my truck. Low end is lower and high end is higher. 7th gear in the 8 speed/3.21 is pretty close to 5th gear in the 6 speed/3.55 combo.

I'll be very interested to hear if peoples impressions line up with the calculations. A hemi with the 8 speed and 3.21 should feel snappier overall than a hemi with the 6 speed and 3.55.

To read the chart, along the top is RPM and down the left are the different gear ratios for the two transmissions. The left hand group shows the tire rotations/minute at a particular engine RPM and the right hand group shows MPH at a particular engine RPM.

For example, at 1500 RPM, you should be going about 68 MPH with the 8 speed/3.21 and about 61 MPH with the 6 speed/3.55 combo.

Edit: I've updated the chart to include all combos of rear and transmission. Interesting, the 8 speed with the 3.21 has a lower low than the 6 speed with the 3.92 and 7th gear is roughly equivalent to 6th in the 6 speed.
PS: I highlighted in green for between 54 MPH and 66 MPH which is where most of us would run on the highway when towing.

The only reason I can see for Ram downgrading the tow rating of the 8/3.21 combo is the tall end ratio of 8th. But given that 7th in the 8 speed/3.21 will run pretty close to 6th in the 6 speed/3.92, real world performance is going to be just as good with the 8 speed/3.21 combo, just tow in 7th gear on the highway.
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Old 05-18-2013, 12:31 PM
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Quote:
Originally Posted by Ddaaryl View Post
I have had both rear end ratios and see no difference in mileage. The window sticker rates them both the same as well
I thought a 4x2 3:55 6-speed is rated at a max of 20MPG HWY and the 3:21 4x2 8-speed is rated at 23MPG HWY? Both with the Hemi of course.
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Last edited by Arth; 05-18-2013 at 12:36 PM.
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Old 05-18-2013, 02:00 PM
dodgetank dodgetank is offline
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Quote:
Originally Posted by mdragon9 View Post
PS: I highlighted in green for between 54 MPH and 66 MPH which is where most of us would run on the highway when towing.

The only reason I can see for Ram downgrading the tow rating of the 8/3.21 combo is the tall end ratio of 8th. But given that 7th in the 8 speed/3.21 will run pretty close to 6th in the 6 speed/3.92, real world performance is going to be just as good with the 8 speed/3.21 combo, just tow in 7th gear on the highway.
Hello! This is my first post here. I just bought a 2013 ram sport 8spd with the "3.55" rear end. After reading about a bunch of trucks getting shipped with 3.21's, I crawled underneath and sure enough, i've been screwed.

I was just wondering if the 3.21 rear end is weaker than the 3.55 or 3.92? Or maybe towing heavy with the 3.21 would put too much stress on the tranny?? From what you posted, it really seems like they shouldn't have downgraded the tow rating for this combo.

Before I attempt to get the stealership to give me what I paid for, I just want to make sure it's the best move. I like the 1400-1500 RPM on the highway, but I don't like the idea of over taxing my tranny or causing damage to a weak rear end.
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