My truck came stock with a 3.55 posi, but I'm thinking that I'll probably need to upgrade to 3.92s when I get my 33s (I'd go with 4.10s, but the dodge website lists 3.92 as the lowest ratio available and I want the dealer to be able to re-calibrate the speedo for me). All I can find online is ring & pinion sets...Will they work with the posi unit that came with my stock gears, or do I need a whole new unit, gears and posi and all? How much will this cost? Thanks.

Also, the truck will almost never be driven off road...is it REALLY necessary to go with lower gears? The most off-road it will ever see is my yard when I haul logs and other junk around.

 

3.55's just suck, plain and simple. The only reason they are on there at all is for emissions ratings (lower rpms at a given speed yield lower emissions) and better fuel economy (although you only see better economy with them in a light 2wd reg. cab driving on exclusively flat terrain).
So, I would say, if you can afford it, redo the gears. 3.92 would be minimum for 33" tires, 4.10's would be better, but the 3.92's would still be night & day over 3.55's.
If you are going to have someone in there for the gears, then replace the stock LSD! It is total crap, not only yielding **** performance, but it will blow up on you some day and take your nice new gears out with it! Opt for an Auburn Gear High Performance Series LSD, way better than stock and should last the life of the truck.

 

Unlike other makes of vehicles you do not have to recalibrate for a gear swap. Get 4.56's, you'll love them!!! Yes the LSD will work with aftermarket gears. What year is your truck? The 06's and above tend to not have the LSD problem the 03-05's had.

 

I'd say to go 4.56's. You'll be impressed by the change. The gears will gain you mileage in town, and eventually be even with your stockers at about 70 mph. If you drive a lot over 80-85 you will see a mileage drop though. Also like helopilot said, gears on our trucks do not affect your speedo, only tire height changes do. [/align]

 

Not that I'm disagreeing or anything, but how can a ratio change not effect the speedo, but a tire height does? Don't they both affect the speedo, or is there a wheel speed sensor instead of the traditional transmission mounted thing? If that really is the case then I'll definately go for a 4.xx ratio.

 

Also, is there any aftermarket solution to recalibrating the speedometer for larger tires that does not involve splicing wires into the factory harness? Like a OBDII port unit available for F bodies and similar vehicles? I'd rather get it done at the same time as the tires are put on instead of dealing with the stealership hassle.

Edit:

I take it I'll need this: http://www.4wheelparts.com/PDT263540.aspx

in addition to a ring & pinion set? Any idea what a ballpark figure is on labor to install the unit w/ the gears? Anyone in the south Atlanta area know of a place?

 

The VSS is in the differential. The computer only measures how many revolutions your axle is moving, that's why a gear change doesn't affect the speed, but a tire size change does.

You can get a Superchips or other tuner that will let you adjust tire sizes.

As far as the LSD, here is the best price you will find an Auburn for.

http://www.rearendparts.com/product....=90&page=1

 

Thanks, I guess I'm out of luck for the tire size recalibration. The only chip I've found for the 4.7 is a Jet and I don't believe it lets you adjust anything.

 

As helopilot notes it's not gear ratio but tire size changes that requires a spedo recal..It's a quick thing for the dealership, should only cost a little (wish they'd do stuff like that as a courtesy though). I think my dealership charged me $45 to do it and it took them all of 10 minutes. You might want to look up the spec's on your tires - if the computer at the dealership doesn't have the exact size they'll need the revolutions per mile. Most tire manuf's will have this...for example Toyo provides them in a PDF (see http://marktg.toyotires.com/file/18971.pdf). This is what I gave them as they didn't have my size (285 X 75 X 17)....you don't have to buy a chip or programer. Though they give you other stuff, but that's about $400 or so more than you need to pay just to recal. the spedo...

 

The dealer can flash your pcm and recalibrate for tire size. I have seen guys say they have not been charged and I have seen guys say the dealer gave them a price of over $100. Average is about $50. You'll have to check with your dealer.