Why I’m Dropping from 33″ to 31″ Tyres on My Jimny

1. Three Chains, Four Years—Enough’s Enough

I’ve owned the little Zook for four years, and I’m already on transfer-case chain number four. The first died on 235/75s, the next two on 33-inch mud-terrains. Each time the chain stretched, it began to buzz under load and finally skipped teeth—instant loss of drive and a whole lot of trail-side cursing. Forum old-hands have been saying for years that the chain is a consumable, especially once you’re above a 30-inch tyre, and they’re absolutely right. BigJimnyBigJimny

1.1 What actually fails in the case?

The Hy-Vo chain links the input and output shafts. Every extra millimetre of tyre radius multiplies the torque seen by that chain. When the pins elongate, backlash grows until the chain tries to climb the sprocket teeth, shearing itself and sometimes the sprockets too. You feel one or two “clunks”, then nothing but limp-mode 2WD—if you’re lucky.

2. How Big Rubber Multiplies Chain Abuse

• Torque lever-arm – My 33s are 10 % taller than stock but weigh far more (see below), so every throttle stab is amplified torque through the chain.

• Unsprung weight – A single 33″ steel wheel + tyre tips the scales at ~24.9 kg versus ~17.4 kg for the factory alloy + HT combo—a 7.5 kg hit per corner, or roughly 30 kg of extra mass the axles and chain must accelerate over bumps teamghettoracing.com.

• Rolling inertia – The tyre carcass alone can be 25–26 kg (BFGoodrich KM3 33×10.50R15 is 56.7 lb / 25.7 kg) before you even add a heavy rim tiresize.com.

• Heat – On long climbs the transfer case oil thins out sooner, starving that already-overworked chain.

3. Touring Reality: 31s Beat 33s on the Open Road

3.1 Fuel, gearing and highway manners

• Economy – Dropping from 33s to 31s nets me an extra 2–4 mpg on motorway slogs—crucial when the nearest pump is 400 km away.

• Gearing sweet-spot – With 33s I lived in 4th and dreaded head-winds; 31s let me hold 5th on mild grades yet still clear ruts the stockers never dreamed of.

• Speedo/odo accuracy – The 31s keep the speedo only ~6 % optimistic instead of ~12 %, so range calculations are less guesswork on desert crossings.

3.2 Unsprung weight, spares and packing

Every kilo I pull off the wheels is a kilo I can put into water, fuel or camera gear. At ~20 kg each my new 31″ all-terrains are still heavier than stock but save roughly 5 kg per corner compared with the 33s—far kinder to wheel bearings and easier to muscle onto the roof rack at day’s end.

4. Why I’m Parking the SJ-Box Idea—for Now

I flirted with the classic SJ410/SJ413 transfer-case swap, but good 410 boxes (the ones that give the right Rock-Lobster ratios) are getting rare and pricey; parts support is patchy at best BigJimny. Until I own a proper tow-rig—so the Jimny doesn’t have to pull itself hundreds of highway kilometres—I’m shelving that conversion. The search for a suitable box (and spare gears) continues in the background while I bank cash for the big build.

5. The Long Game: Tow-Rig + Rock-Lobster + 35s

• Phase-1 (now) – 31s, stock chain-drive case, conservative throttle: reliable daily-driven overlander.

• Phase-2 – Buy a tow-rig with a beefy gear-drive transfer case that can haul the Jimny to remote trailheads.

• Phase-3 – Drop in Rock-Lobster gearing, 35s, chromoly shafts and lockers. At that point the Jimny becomes a dedicated rock-crawler that can drive on-road, but only to reach the next trailhead.

6. Key Take-aways

1. Transfer-case chains hate leverage—33s accelerate stretch and failure.

2. 31s are a strategic retreat: less stress, better economy, still plenty of clearance.

3. SJ/Rock-Lobster swap is on hold until I have a tow-rig and parts inventory.

4. Long-term vision: two-rig setup—one for miles, one for rocks.

No comments section here—but if you’ve cracked a bullet-proof chain solution, hit me up on Insta or the Jimny forums. This chapter is just getting started!