Oaky now is the time to upgrade my rear end a bit.....considering my pinion and spyder gears are in peices. Okay it is a 97 Dakota Sport with R/T wheels 255/50/17. Might go larger in the future but not by much. It is the 3.9 Supercharged with 42re Auto tranny, 8.25 Open with 3.5 gears (According to my Glovebox sticker). I probably should find out my gears for certain but lets assume right now they are stock. I do a lot of highway driving with some city driving (Phoenix, so lots of traffic). I am looking for a good combo to do good on the strip but also on the road course and of course something that will not hurt my gas mileage too too much (I know i want it all). I am going to put a LSD in it since i tore up the diff also, not sure which one just yet. What do you guys recommend?

 

I wouldn't go too short in gearing, considering the torque characteristics of these engines (mostly made in the lower 2/3rds of the RPM range). I think either a 3.55:1 or a 3.92:1 gives a fairly good mix of performance. I have the 3.55:1 gears in my '97, and my tires are quite a bit taller than yours (29" tall vs. 27" tall), and I still think a 3.55:1 is short enough. I probably wouldn't go shorter than 3.92:1.

 

That is what i was thinking. I was looking at the tire sizing i might go to and it is around 28" to 28.5". So, and i do a lot of highway driving also. Do you know of a good calculator to determine my highway speed at certain RPM?

 

you may just want to think about swapping the whole rear for an R/T rear end. that way you get a stronger rear, 3.92's and a factory LSD.

 

Dont R/T's come with all wheel disc too, that would be nice. Also dont those weigh a bit more?

 

i think only 03 R/T's came with disc brakes in the rear, but im not sure. and yes the rear is a little heavier but it's stronger, something to think about if you're going to be doing burnouts and racing, which if your 3.9 really is supercharged... you better be.

 

I agree with swapping an entire rear axle into it. The 9.25 came in non-R/T models also, after about 2000 or so (I don't know the actual break point).

Here's a good speed calculator: http://www.kabamus.com/garage/gears.html

 

What about an 8.25 with an Auburn Posi and 3.92's? I ask because i just found one here in PHX.

 

Sounds fine to me. I don't know much about the reliability (or not) of Auburn units, but an 8.25" axle with 3.92s should be just fine. Especially if you can get the whole thing for a couple hundred bucks.

 

Ya, he is debating selling it to me or not. Either way i will go with the R/T rear or an upgraded 8.25. Thanks for all your info and advice.