I been tossing the Idea around of Changing my rear end gears From 3.21 to 3.90's or 4.10"s I dont know whats the best way to go I could use other Dodge truck ownners two cents on this, As of now I turnning 2100 RPM"s at 70 MPH.

I have a 2006 Dodge 1500 Q-Cab with a underpowered 3.7 V-6 dont Laugh it was a good idea at the time.

It has 6 Speed Maual trans I do mostly HWY Driving posted speed is 70 MPH round trip to work and back is 65 miles a day lots of wind and rolling hils the truck has no As!! it needs some lowwer gears to get it to wake up and not be so lazy, but how low to go I am running stock size tires 245-70 I think -17 for now mite go with some 295-55-20 one day.

I tow may be 700 to 800 LBS once in a while.

On the weekends also mostly HWY miles we drive 200 to 300 miles per week end that why I gotten the 3.7 this time I thought it would get better MPG I wish I gotten the Hemi insteasd about the same MPG what I been reading Oh well live and learn.

Oh yea I asked this Question on the V-6 Forum no Feed back from them.

 

i would go with the 3.90's just for the fact of what RPM your turning now with the stock gears and you should still beable to pull some "half *** decent" mileage with em

 

Try this calculator

http://www.4lo.com/calc/gearratio.htm

 

What do you think about 3.73 gears not enought or go with the 3.90's.

 

I'd go 4.10's. The gas mileage difference between the 3.92's & 4.10's would be negligable, but you would feel a little bit of difference in acceleration. Better towing performance also. If you do indeed plan on larger tires down the road, you will definately wish you'd have gotten the 4.10's then.

 

Since you have a manual transmission
I am confused why you don't downshift to a lower gear
to drive.

Your truck does not 'know' what diff gear you have
or what gear you are in,
it only performs due to the overall gear ratio.

Switching to high numerical diff gears will just make
your first gear much lower...which is useful to off-road crawling
but not of any use to on-highway use.

 

HankL,
3.21 are so bad, you would have to drive them to know what we are talking about. To move from dead stop is horrible. 3.21 should be banned from full size trucks unless your house is next to the highway and your work is right off the same highway.

 

I changed the stock 3.55 diff gears out to 3.21 gears years ago.at 44,000 miles..on a 1995 model 5.9V8 and gained one MPG at 70 mph and still drive it every day with about 190,000 miles now. Is that enough experience?

I have a set of 4.10 and a set of 2.96 diff gears for a 9.25
(both given to me free)
in storage along with the original 3.55 gears
but the 9.25 has enough reliability problems that it is not worth wasting too much time on.

I have driven the Ram 6 speed Getrag (which has a clutch I don't like)
with the 3rd Gen Ram and 3.7 V6 several times.

Look at the ratios in the light duty 6 speed and calculate the percent difference between gears:

1st - 4.23
2nd - 2.53
3rd - 1.67
4th - 1.23
5th - 1.00
6th - 0.79

A WAG'er would advise someone with 3.21 diffs to spend the money to swap to higher gears, but someone with just a bit of practicality would point out he can find out how that would feel by simply downshifting.

 

Does your 5.9 V8 have a overdrive like the 6th gear on our 3.7? I have done the numbers and to be honest I drive 70mph for about 15-20 minutes, but the other 30 minutes tops is 45mph, my rpms wont be that high at 70 to cause severe mpg loss if any.
What I am trying to understand from what you are saying, by switching gears from 3.21 to 3.92 or 4.10, it is NOT going to help me move quicker? meaning from dead stop, I will not see ANY difference? Because that is what we(I believe)want.

PS- I will gladly accept your 4.10 in storage for the same amount you paid(0) plus shippingif you dont want them at all.

 

HankL?