8 spd 3.21 vs 6 spd 3.92

magic280z said:

There are some definite opinions on here about 3.92 gears. Having just purchased a 3.21 Hemi 8spd truck they would have you feel that the truck can't really do anything and is nothing more than an oversized suv. Now most of us may use our trucks for that doesn't mean they can't also be a truck.

I decided to compare the final drive ratios of both setups and found out some interesting info.

8-spd (8hp70) 3.21
115.09
210.05
36.74
45.36
54.14
63.21
72.7
82.15

6-spd (65rfe) 3.92
111.76
26.55
35.88
43.92
52.94
62.63

Looks like the 8spd 3.21 still has the edge over the 6spd 3.92. I know everyone knew the 8 was better than the 6 all else being the same, but this shows how much different the 8spd is. Basically the 8 spd 2-7 is the 6 spd 1-6. You gain a whole lot better first and pretty decent 8th. And the 6 spd really is just a 5spd during linear upshifting. 3rd gear is an optional kickdown gear or it can be used when electronically shifting, but is probably useless being so close to 2nd.

So if you are upgrading from a 6 spd 3.92 truck you should feel comfortable going with the 3.21 gearing you will have better acceleration and fuel economy. If you just want the fastest possible acceleration or need to tow 10,000 lbs then feel free to go 3.92 with the 8spd or go find an r/t with a 4.10 (not sure if 8spd or 6spd).

I added the 3.92 just to have the info.

8-spd (8hp70) 3.92
118.42
212.27
38.23
46.55
55.06
63.92
73.29
82.63

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crash68 said:

This proves you need to learn how to do math.
How can 3.21 and 3.92 be so close together? There is a 20% difference between the two rear axle ratios, none of your numbers reflect that.

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charonblk07 said:

I don't know how you can disagree with the numbers when looking at the final output drive ratio comparison. The differential gear ratio is the constant and the trans gear is the variable, all you do is multiply the trans gear ratio by the differential ratio to get the final drive ratio, no sure where the problem is. Tire diameter will make a difference to what is put to the ground but what's generated by the drivetrain will remain constant regardless of wheel diameter and the difference will be the same on a vehicle with a constant tire diameter but different differential ratios. Here are the 8 speed trans gear ratios, feel free to try your alternative mathematics and let us know what you get.

4.714
3.143
2.106
1.667
1.285
1.000
0.839
0.667

I attached what I got, let's compare; the last colum is the differential percentage, look pretty close to 20% for me.

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crash68 said:

I think your mistaking magic280z numbers, I didn't do any of that math. I quoted what he did and pointed out that he couldn't do math. His 3.21 and 3.92 numbers do not make any sense, they are not 20% apart, nor are they even close to the actual numbers.

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THETANK said:

^^^mine to and am I the only one that does not stress about gears ie I have 3:92's but the way I drive they might as weel be 2:38's.

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crash68 said:

This proves you need to learn how to do math.
How can 3.21 and 3.92 be so close together? There is a 20% difference between the two rear axle ratios, none of your numbers reflect that.

Click to expand...

69GWC said:

Anyone have thr numbers on the 6sp in the 2500s

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magic280z said:

There are some definite opinions on here about 3.92 gears. Having just purchased a 3.21 Hemi 8spd truck they would have you feel that the truck can't really do anything and is nothing more than an oversized suv. Now most of us may use our trucks for that doesn't mean they can't also be a truck.

I decided to compare the final drive ratios of both setups and found out some interesting info.

8-spd (8hp70) 3.21
1 15.09
2 10.05
3 6.74
4 5.36
5 4.14
6 3.21
7 2.7
8 2.15

6-spd (65rfe) 3.92
1 11.76
2 6.55
3 5.88
4 3.92
5 2.94
6 2.63

Looks like the 8spd 3.21 still has the edge over the 6spd 3.92. I know everyone knew the 8 was better than the 6 all else being the same, but this shows how much different the 8spd is. Basically the 8 spd 2-7 is the 6 spd 1-6. You gain a whole lot better first and pretty decent 8th. And the 6 spd really is just a 5spd during linear upshifting. 3rd gear is an optional kickdown gear or it can be used when electronically shifting, but is probably useless being so close to 2nd.

So if you are upgrading from a 6 spd 3.92 truck you should feel comfortable going with the 3.21 gearing you will have better acceleration and fuel economy. If you just want the fastest possible acceleration or need to tow 10,000 lbs then feel free to go 3.92 with the 8spd or go find an r/t with a 4.10 (not sure if 8spd or 6spd).

I added the 3.92 just to have the info.

8-spd (8hp70) 3.92
1 18.42
2 12.27
3 8.23
4 6.55
5 5.06
6 3.92
7 3.29
8 2.63

Click to expand...

charonblk07 said:

I don't know how you can disagree with the numbers when looking at the final output drive ratio comparison.

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blrmkrl83 said:

His numbers look close to me. The 6 speed was right on, but the 8 speed was .01 to .02 off from what I got. The place we found the trans ratios must be off just a bit or rounded different.

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69GWC said:

66rfe gears

1st 3.231
2nd 1.837
3rd 1.410
4th 1.000
5th 0.816
6th 0.625
Reverse 4.444


does that seem right, the 6sp gearing on the 2500s. it looks like the 8sp 6th gear is about the same as the 66 rfes 1st gear.

I guess the final gear ratio need to figured in.

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crash68 said:

Here in lies the problem, a number format issue with Tapatalk, the spacing between the numbers.

If they were typed with some sort of list format like 1:

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ExpressRules said:

I agree about why stress about gears. And why do these types of threads have to turn into a who can **** the furthest contest?

I'm very happy with my 3.55 and when I do need to get into it the extra 1/8 of a second the 3.92 would have gotten me there is of no matter.

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magic280z said:

Sorry edited post to separate gear and ratio. These are final drive gear x diff* all produced side by side*using a gearing website. There are multiple ratios for the 8hp70. I couldn't find the exact ram ratios. These was the lowest ones I found.

When I purchased my truck the 3.55 was only available on the v6. Didn't see it for the hemi.

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magic280z said:

There are some definite opinions on here about 3.92 gears. Having just purchased a 3.21 Hemi 8spd truck they would have you feel that the truck can't really do anything and is nothing more than an oversized suv. Now most of us may use our trucks for that doesn't mean they can't also be a truck.

I decided to compare the final drive ratios of both setups and found out some interesting info.

8-spd (8hp70) 3.21
1- 15.09
2- 10.05
3- 6.74
4- 5.36
5- 4.14
6- 3.21
7- 2.7
8- 2.15

6-spd (65rfe) 3.92
1- 11.76
2- 6.55
3- 5.88
4- 3.92
5- 2.94
6- 2.63

Looks like the 8spd 3.21 still has the edge over the 6spd 3.92. I know everyone knew the 8 was better than the 6 all else being the same, but this shows how much different the 8spd is. Basically the 8 spd 2-7 is the 6 spd 1-6. You gain a whole lot better first and pretty decent 8th. And the 6 spd really is just a 5spd during linear upshifting. 3rd gear is an optional kickdown gear or it can be used when electronically shifting, but is probably useless being so close to 2nd.

So if you are upgrading from a 6 spd 3.92 truck you should feel comfortable going with the 3.21 gearing you will have better acceleration and fuel economy. If you just want the fastest possible acceleration or need to tow 10,000 lbs then feel free to go 3.92 with the 8spd or go find an r/t with a 4.10 (not sure if 8spd or 6spd).

I added the 3.92 just to have the info.

8-spd (8hp70) 3.92
1- 18.42
2- 12.27
3- 8.23
4- 6.55
5- 5.06
6- 3.92
7- 3.29
8- 2.63

Click to expand...