Thats your best bet haha

 

i'd go 2.5 inch with the muffler of your choosing. i have the thrush welded and i love it. i didn't put 3.21 gears in it. that's what the original owner orderd it with. it had duels when i got it and i chucked those (rotted) and went back to the 2.5 inch pipe so that i could get my power back in the low rpm range. about 2200 to 4800 rpm that thing will pull like hell. below that it pulls really well. i can cruise in the mountains at 55 (wich is about 1200 rpm) and only on long very steep pulls does it come out of over drive. i would love to have 3.92's but the cost is to much with the other things i have to fix right now. with 3.92's i'd have 3 inch pipes.

 

Explain your logic. Why would you have a bigger exhaust with higher gears? Gears only changes towing/pulling power and acceleration. The amount of exhaust flow wouldn't change. 4800 RPM is 4800 RPM regardless of what gears you run with.

 

with 3.92's you would be running higher rpm all the time. you'd have the mechanical advantage getting it rolling but when you stomp it and you get it winding out the higher rpm power (more airflow means lower exhaust velocity wich changes where the peak torque is) would take better advantage of it. that is if you are after speed. if you want low end power then the smaller pipes keep the velocity up at lower rpm making it breathe more efficiently at say 1800 rpm wich would be great for towing.
it really depends the most on how you intend to use it.

with the 3.21's i spend most of my time in the lower rpm (2k or less unless im running about 80 with 31 inch tires) so i take best advantage of the low end torque wich keeps it from shifting in and out of overdrive all the time. when i tow i wish i had the 3.92's but i do more highway driving than i used to and the power at 65 when i stomp it and it drops to second is just great and it lasts till 80 before hitting third. for my uses right now that is the setup that is best for me. if i had 3.92's with the way i drive i'd want the 3 inch pipes since i would spend more time above 2k wich would make it more efficient. that would help keep it from feeling like it ran out of power at 4800 rpm and feeling stupid strong at 2k rpm. but for towing again that would be a good thing. for racing not so true. just depends on how you use it.

if you want to read up on it more look up volumetric efficiency. that describes in good detail how the air speed relates to efficiency. to fast and it sucks out the mix while the valve are overlapped and kills top end but does awsome at low end. smaller pipes are good for torque. bigger for high rpm because the air speed is low enough not suck out the mixture and help keep it in the cylinders. but bottom end is not as good because the very low speeds at lower rpm it doesn't clear the exhaust from the cylinder as well.

half an inch doesn't sound like much but i could feel it pulling hills and towing. and like i said at 65 i can nail and it pulls like first gear till it hits about 80 then third still pulls pretty strong. i don't know when third runs out though. im not brave enough to go that far with it and i haven't done the math. for me the gears dictate i need low end power to take advantage of the higher speeds it can run.
if i wanted faster full throttle acceleration the i would want bigger pipes and 3.92's.

 

Lol I seemed to have forgotten what higher gears do at higher speeds. Derp.

 

no problems at all. i can't count how often i've have lapses or forgot details (try rushing a engine job and relizing when you try to leave the stall that the shifter wasn't hooked up or the solenoid pack isn't plugged in).

if only we had as fast and reliable memory as our computers i'd be out of work :P

 

Gotta question . . . if we go from 3.55 to 3.9 gears, can we use the same carrier and is it the same carrier for sure grip or non sure grip? Thanks

 

Yes you can use the same carrier.

 

For the cost you really aren't getting much more with 3.9s.

 

I went from 3.55 to 3.9 with 33's it is defiantly worth it. Ring and pinion $150 www.ringpinion.com easy to do on your own while following the service manual or have a shop do it for $100-200 or so. Also some Dakotas came with 3.9's stock easy axle swap.