Larger tires require numerically higher gear ratios to keep effective drive ratio in the same range. Chart for the 4.0L automatic and manual XJ.
When you go to bigger tires, your effective drive ratio drops (taller tires = each axle revolution travels further). Without re-gearing, the engine bogs in higher gears and the transmission shifts late. Re-gearing restores the drive ratio.
XJ gear ratio guide for the 4.0L:
AUTOMATIC (AW4):
MANUAL (AX5/AX15/NV3550):
The AW4 automatic XJ benefits MORE from re-gearing than the manual because the torque converter loses efficiency with mismatched gearing. A 33" tire XJ with stock 3.55 gears (common) is dramatically improved with 4.56 re-gear.
Gear costs: Yukon gear sets ~$200/pair, master install kit $80, install labor at a gear shop $400-600 per axle. DIY install is doable with patience and the right tools. Pinion bearing preload and ring gear backlash are precision settings.
Both axles must match. If you re-gear, do both axles at once (or order both ratio matched).
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.