i would probbaly get the new ring and pinion if u have the money

 

ok just went out in the rain to get the tire size, they are 225/75R16.

 

Yeah but he does have a v6 so... His shifts would be crazy low lol, almost stalling low...

 

here's a simple way to look at it.

3.21 to 3.55 is only a 10.6% gear reduction, so to keep the same tires, and the same speed, your engine rpm would go up by that amount. so 2000 rpm would climb to 2200. that's not very much. for that back to school feeling divide 3.55/3.21 = 1.105 (drop the 1).

3.21 to 3.90 is 21.5. that's a significant change to keep the same tires. 2000 rpm would become 2400. still not too bad.

now it gets complicated. if you put on bigger tires, that will reduce the gear drop by another percentage.
225/75/16=29.28 inch. 285/75/16=32.83 inch. bigger/smaller = 12%.
http://www.1010tires.com/tiresizecalculator.asp

so if you combined a 3.21->3.90 gear change (-21.5) with a 225->285 tire change (+12), then the result is 21-12=9 (9% reduction).

here' what i would do....
1. find a complete rear axle assy and do the swap yourself. it'll be cheaper than changing the gears, and you'll enjoy the work (maybe). avoid swapping the ring and pinion gears, as its a pita. i think any year will work, but measure! and check the drive shaft yoke attachment. some use a u-joint yoke, later years used a flange. you might need the drive shaft...
2. go with the 3.90. your initial change will be huge (-21%).
3. swap to bigger tires in the future. you know you want them. it will offset the gear change.

edit- standard junkyard pricing for axle assembly is around $350.



for you 3.55 guys.
gear 3.55 -> 4.1 is -15.5%
tire 225 -> 285 is +12 15.5 - 12 = 3% overall gear drop. haha not much you're barely back to stock.

gear 3.55 -> 4.56 is -28.5%
tire 225 -> 285 is +12 28.5 - 12 = 16% overall gear drop. significant drop.

 

If you replace the entire axle, and do the work yourself, you can save a bit of money there. Just pull the cover, and see what you find in there. Doing up the brakes at this point wouldn't be a bad plan either. (and maybe doing the 1-ton slave cylinder upgrade?) 3.55 axles should be a dime a dozen....... It was rather common.

Welcome to DF!

 

Thanks for the information, greatly appreciated!!

Alright I am little confused about the gear ratio to select now. DHVaughan made some very good points that I have to mull over.

I would enjoy the swap but am not sure about the shock absorbers though, never dealt with those in my life and not sure how to retain them so they don't fully extend and then to get back into place. Is there a tool that can make this job simpler and safer?

Another question in regards to dhvaughan reply, can one assume the torque increase would be the same as the difference between the different ratios. I.E. 3.21->3.9 is a 21% jump. Would that translate into a 21% increase in torque, assuming same tires and gear (4 vs. 5th gear)?

 

shocks. edit - answered a couple of threads down.

what actually hold the axle assy in place is the u-bolts that clamp the axle to the center of the spring.
other things to remove is speed sensor wire on top of the diff. the brake lines. emergency brake cable. brakes, wheels, tires.

 

I would certainly go with 3.92s, the 3.9 will appreciate it.

 

sorry - just reread the shock question again. just ignore them, push them out of the way and let them extend all the way (if they will). it won't hurt them. when reinstalling, if you can't push them in place by hand, slide a floor jack under them and compress them, then manhandle them into place. no special tool. if they're old, its unlikely they are very stiff. might need replacing.

 

Update - Well I never did end up performing the axle swap. The turck is my fathers and he didn't want to spend the money only to get bad MPG's. After further educating myself on axle ratios, I have realized that an axle swap might improve MPG's and performance over the 3.21 rear end especially since he pulls trailers with this thing 40% of the time. Granted they aren't large trailers but still towing is hard on this setup. I currently have the truck again but over the winter he put on 265/75 r16's (free used tires) on the back and the thing is even worse(performance) then when it had the 225/75R16's on it. It now has hardly any acceleration doesn't matter if you floor the thing, still only accelerates at one speed.

Still thinking about swapping though...