Well, here's what happens. The 30 inch tires are 86% smaller than the 35 inch tires. What that means is that the effective gear ratio becomes 3.73 * 0.86, or 3.21. Which is very tall gearing for a manual-transmission Jeep LJ. Frankly, the stock gearing was already a little tall in 6th gear, I had to downshift every time I hit a hill, this would make it horrible indeed. How horrible? 6th gear in a Chrysler NSG370 manual transmission is a 0.83 overdrive. Using this gear calculator, at 75mph I'd be at 2256 rpm, rendering 6th gear useless for pretty much anything slower or on any slope steeper than my bedroom floor. Whereas with the stock 30 inch tires, I'd be at 2632 rpm, which is much closer to the torque peak of the venerable old 4.0L AMC I-6. 235 lb-ft @ 3,200 rpm. But more important, my crawl speed at idle (1000 rpm) with 2.71 low range transfer case and 4.46 1st gear ratio goes from 1.98mph to 2.31mph -- which is significant when you're rock-crawling, where slower is better. And the 1st gear mph at 1000rpm in 2hi (1:0 transfer case ratio) is 6.26mph, which would make it hard to start off without slipping the clutch as vs. the 5.37mph of the stock tires. Not Good.
So I can upgrade to 35 inch tires, but to do that, I have to go to lower gearing (higher multiplication ratio). My Jeep LJ is equipped with the Dana 30 axle. This is a relatively small axle that has only three gearsets available that are lower than the 3.73 in my Jeep: 4.10, 4.56, and 4.88. Because of the small size of the axle housing, the pinion for 4.88 is *very* small -- Dana only recommends 4.56 as the maximum. So I plug 4.56 into the gear calculator to compute 1st gear mph at 1000 rpm and... 1.88mph crawl speed in 1st at 1000 in 4-lo, or slightly lower than OEM. At 75mph, I'd be going at 2758rpm as vs. 2632rpm with the stock gearing. Not a big deal, it'd make 6th gear more usable in less-than-flat terrain though I'd still have to downshift for big hills.
Now, why am I talking about gears? Because I'm thinking about putting a ARB Air Locker in my rear end for better traction off-road. The stock limited slip has very little friction material and is slowly losing its capability to provide any traction benefit at all in offroad situations. Thing is, this replaces the stock spider gear carrier with the ARB spider carrier, and requires complete gear setup as a result. Gear setup on a Dana 44 rear axle is a PITA, requiring you to set pinion depth with a crush sleeve and a 250 ft/lb torque wrench, set pinion bearing preload with a 15 inch-pound torque wrench and spacers prior to setting pinion depth, spread the case with a case spreader and use shims on the insides of the bearings to properly preload the carrier bearings, and adjust the shims on the inside of the carrier bearings side to side to properly adjust the side-to-side gear lash. It can take a couple of hours and a lot of cursing to get all the depths and preloads and everything right (and remember that the shims are on the *insides* of the bearings and the bearings are pressed on and pulled off with a bearing puller), and people justifiably charge an arm and a leg to do it. So when I have someone put in the ARB, it makes sense to do the gear setup for the 4.56 gears at the same time. That way they only have to do the gear setup *once*, not *twice* (once for the 3.73, once for the 4.56), which would save a lot of time and hassle. And remember, when you're talking about wrenching, time and hassle is $$$.
So anyhow, that leaves one last problem. Right now I have 32" tires on my Jeep. They still have at least 20,000 miles of wear left on them before I am ready to replace them (i.e., until the tread depth gets low enough to start affecting offroad traction). What happens if I put 4.56 gears on the Jeep before I put the 35" tires? Well... rpm at 75mph goes to 3016 rpm. As vs. 2937 rpm if I am running 5th gear (1:0 ration) with 32 inch tires at 75mph. So I tested 5th gear at 75mph, and it's a bit busier than 2600 rpm was with the stock tires and gearing, but still acceptable. Note that I never -- ever -- go faster than 75mph in my Jeep. It just isn't what a Jeep is made for. So I could go ahead and get my Jeep re-geared at the same time I get the locker put into the rear, and I'll still be fine, it's just that my Jeep will be a bit buzzy (but not excessively so -- 3000 rpm isn't all that high).
Now comes the final issue: Should I pay to have someone do the locker install and gear setup, or should I do it myself? Well, here's the deal. To do it myself, I'd need the following spendy stuff: A 0-30 in/lb torque wrench. A hydraulic press. A 500 ft/lb torque wrench. A case spreader. Probably a couple other things I've forgotten, while adding up the above $500+ worth of tools (all by itself) which is more than I'd pay for the gear setup. Given that we're talking about a job that'd take me several days, I'm taking names of any competent axle people you know in the San Francisco Bay area. Heck, anywhere else within reasonable driving distance, for that matter... I'd rather pay to have it done right but have to drive 250 miles to get there, than to pay someone local who screws it up and then I end up with the axle tearing apart and needing to buy a new axle. Bummer, big time, dude!
-- Badtux the Jeepin' Penguin