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.
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:
Why not:
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.
| Part | Vendor | Est. price |
|---|---|---|
| Klune-V 4:1 underdrive | Klune Industries | ~$2200 |
| Tera-Low 4:1 kit (used) | used market | ~$800 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.