Crawl Box Add-On (Tera 4:1 / Klune-V)

Difficulty 5/512–30 hrs$1800–35001987-1990, 1991-1995, 1996, 1997-2001

Short answer

Bolts to the back of an existing trans or front of t-case to multiply low range. Klune-V 4:1 turns a 2.72 NP231 into ~10.9:1 effective low. Crawl ratio paradise.

Underside view of a driveshaft and U-joint running into a rear differential between two mufflers — Photo: Wesley Tingey on Unsplash

A crawl box is a separate reduction unit that mounts between the transmission and transfer case. It adds a fixed low range that multiplies with the existing t-case low. Two main products:

Klune-V (4:1 or 2.7:1 versions, $1800-2400): aluminum case, gear-driven, twin-stick optional. Pair with NP231 2.72:1 low for a combined 10.88:1 reduction. Add to a 4.56 axle and 32" tire, your crawl ratio gets ridiculous (>110:1).

Tera-Low (legacy product, increasingly hard to find new): 4:1 internal reduction kit that fits inside the NP231 instead of mounting separately. Different concept — same crawl ratio gain without adding length. Used to be the budget choice. Tera-low has been discontinued in current production but parts exist on the used market.

Why add a crawl box vs swap to Atlas:

Cheaper than full Atlas
Keeps the existing NP231 (parts familiarity)
Twin-stick option without case swap
Stack with later upgrades

Why not:

Adds length: driveshaft must be shortened, may need crossmember mod
Adds another fluid service point
Adds weight
Klune-V 4:1 is loud at speed if engaged unintentionally

For most serious XJ builds, Atlas II is cleaner. Klune-V is an option for builds with existing NP231 they want to keep, or for stacking with an Atlas to get insane crawl ratios.

Why it works

massive low range
keep existing t-case
modular

Trade-offs

length added
expensive
rare parts on Tera

Tools required

full shop
trans jack
welder for crossmember mod

Parts

Part	Vendor	Est. price
Klune-V 4:1 underdrive	Klune Industries	~$2200
Tera-Low 4:1 kit (used)	used market	~$800

Sources

Klune-V product

Written and maintained by an AZ wheeler and driveway wrencher. How we source guides. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.