The rear end is only half the equation, final drive ratio which includes transmission gearing is what matters. If you have a lower gear ratio then you'll take off quicker but you will top out at a lower top speed. Rams are limited to 105 MPH anyway, but you can remove the limiter with a tuner. IMHO 4.88s with an 8 speed are unnecessarily low and will give you a 22.98:1 1st gear final drive ratio-you will probably shift out of 1st by the time you hit 3 MPH. If you have 3.92s i wouldn't waste money going any lower.
Many people are not used to newer transmissions that have a lot of gears and allow for a wider range. Back in the day when they had 3 or 4 speed transmissions you needed 5.88s to get a load moving with a tall 1st gear in the transmission, but that also limited your top speed. By gearing with the transmission you have a similar top gear (8th and 6th gear for the 8HP70 and 65RFE respectively are both 0.67:1), but a lower first gear (4.71:1 in the 8 speed vs 3.00:1 in the 6 speed).
If you had a 6 speed with the same final drive ratio in 1st gear that the 8 speed provides then your diff gears would be:
3.21 (8 speed): 5.03 (6 speed) differential
3.55 (8 speed): 5.57 (6 speed) differential
3.92 (8 speed): 6.15 (6 speed) differential
conversely, an 8 speed would have to have the following diff gears to provide a similar final drive ratio to the 6 speed:
3.21 (6 speed): 2.044 (8 speed)
3.55 (6 speed): 2.26 (8 speed)
3.92 (6 speed): 2.50 (8 speed)
Actual 1st gear final drive ratios for a given differential/transmission combo:
3.21/6: 9.63:1
3.21/8: 15.12:1
3.55/6: 10.65:1
3.55/8: 16.72:1
3.92/6: 11.76:1
3.92/8: 18.46:1
4.10/6: 12.30:1
4.10/8 (never offered, but demonstrative): 19.311:1
When have 5.03s ever been a necessary gear in a 1/2 ton? You can only get so far before you end up with such low gearing that the truck shifts to 2nd at 2mph or so. How useful is that gear?
Once you are moving and out of 1st gear it doesn't make much difference anyway as a truck with taller diff gears can drop a trans gear to accommodate. FWIW the trans gears for the 6 and 8 are, respectively:
1st 3.00:1
2nd 1.67:1
3rd 1.50:1
4th 1.00:1
5th 0.75:1
6th 0.67:1
Reverse 3.00:1
1st 4.71:1
2nd 3.14:1
3rd 2.10:1
4th 1.67:1
5th 1.29:1
6th 1.001:1
7th 0.84:1
8th 0.67:1
Reverse 3.30:1
Now compare this to Toyota, which I will use as an example because they offer only 4.10 or 4.30 gears in the Tundra which are very low. This is because their transmission gearing is so tall. With the 4.30 diff in a Tundra and their 3.33:1 1st gear you get a 1st gear final drive of 14.319:1
A Ram with the 8 speed and 3.21s has a first gear final drive of 15.12:1 as I said before.
That means if you have an 8 speed Ram with 3.21s it is actually geared lower than a Tundra with 4.30s.
Another thing, if you are going super low your ring gear will need to be larger or you will risk breaking it, which is one reason Toyota uses a 10.5" ring gear (and you can find accounts of theirs braking more than infrequently). We have 9.25" ring gears, so lower than 4.10 I would be concerned. I don;t know if a 4.56 or lower will actually fit.
Some of this is useless to you, as it was a direct copy paste from a conversation i had with someone else, but the general information is still the exact same. hope that helpswith understanding the gearing in the 8 speed.
