We will be getting new tires for my girlfriends CRD and allthough she would like to go stock, I am trying to talk her into a taller all-terrain tire. How big can we go with a basically stock drive train except for a frankenfurter (that's what she calls it) 2" lift without having to regear ?

First question - how big a tire can you put on it - some folks have done 255/70/16 stock
but with a 2" lift 245/75/16 is common - depending on tread width (not section width) you might have to bend a pinch weld
Tire guide - http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?p=551144#p551144
pinch weld - http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=59&t=7690&hilit=pinch+weld

Second question - gearing - is more a function of how you drive.
If you're not towing - but a lot of highway - probably don't need to regear at all - Diesel torque can deal with a lot of variations, and you can't put a really big tire under a Liberty.

if you're towing especially around mountains or lots of stop and go traffic no highway - you might consider regearing with stock tires.

the target is to get the engine to ~1800 rpm (Torque peak, BSFC low) - for most of your driving, and you don't have many options 3.73 stock, 4.10 or 3.55's are available - (but hard to find for the front)


(and Welcome to LOST)

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2005 CRD
stuff
Skeptic quod gratis asseritur, gratis negatur

X2 on Diesel Torque.
The gearing is not the weak point for the CRD, it is the Original Torque Converter when compromised by a soon to go bad front pump. Chrysler cut the torque in the ECM and changed the shift patterns in the TCM with the F-37. This was a cheap @$$ Bean Counter Engineering stunt to avoid replacing the front pumps and using the 0.200" dampener springs in the torque converter that VM recommended from day one. Now Chrysler makes a new pump and sells the J8/Euro as the replacement torque converter.
As long as you take care of the problem with the weak Original Converter/problem front pump, you can address the ECM issue with a tune.
To fix the TCM problem you either need to either pay a bunch of bucks to find a Pre F-37 TCM, pay $1,200 up front for a custom TCM tune from bgchrysler.com, or wait for a TCM flash under development. I am personally involved with a fix for the F-37ed TCM. To use some legal court room BS, the prosecutor has been assigned, former Illinois Governor Rod Blagojevich will be the assigned as the defense attorney (they shurly will loose), but the Jury is still being selected.

So, if you regear the differentials, you will make the F-37 problem worse unless you put on tires that are so big that you cancel out the ratio change, then you will need a serious lift kit.
Fix the weak point with the front pump and Original Torque converter, get a tune with the torque you want, and wait for a performance TCM flash, you will have a stronger rig.

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2006 Pearl Green CRD
Magnaflow 2 1/2" Cat Back
KJ Extra Leg Room Brackets, Carter Lift Pump, V6 Airbox, ORM
Fuel cooler, Oil Separator, Progard 7
Gauges EGT Boost Trans Temp Oil Pres, Michelin LXT AT2 245 70 R16
7,000# Draw Tight hitch, PML EX Deep Trans Pan
Centrifuge, SunCoast, Transgo, RAM TCM, InMotion Stage 2
Wife's 99 TDI VW Beetle

I have 4.10 gears and 235/85R16 (32") tires I have changed nothing else in the drive train and have gone 65K miles and wheeled coast to cost without a single problem. I did have to change the CV's at 81K however.

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2006 CRD http://www.UtahPowderCoatings.com/ http://www.gorecon.com/ I'm kind of a big deal.
RL rear bumper, ARB front bumper, Mile Marker winch, Power Tank, Skids by Mopar & Skid Row, RL Super Skink Sliders, 235/85R16 MTR, Air Flow Snorkel, DTT, 4.10 gears, Frankenlift II, Katskin leather, JL Audio stereo, Rosen video.

We are going with the Frankenlift (pretty sure) so not gonna get "stupid" with the tire size. Sometime down the road, Suncoast will probably get to work on our future tranny problems, but her critter runs pretty great so it will be a "put as good a part as we can" when it comes time to replace/repair. The 245/75R16 sounds like a good compromise.