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Ok, so I know the idea of gears in general from my very technical training on legos... small gear turning big gear = more torque at a slower speed versus big gear turning small gear = less torque at higher speed.

My blank stare into space though, is how many gears and which feeds into what on the FJ 4x4 system, specifically the 6MT. For gearing with larger tires (like 99.999% of all people re-gearing), I see members swapping for something in the 4.xx. Which gears are these... just the front/rear diff? Or something in the T-case also?

Now, (please dont shoot the messenger), suppose I wanted to go the other way around and drop down from the 3.91? MT to the 3.73 AT gears... is it also just the 2 sets of gears and is literally a swap and bolt on affair?
 

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You will not notice a difference with that small of a gear change. I have 411 and my buddy has 373 and you can't tell the difference. I'm about to move to 513 on my jeep and go 37"

That is a waste of money trust me and will cost you about $1200-$1500 with labor and parts. Dealer could be over $2000
 

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Ok, so I know the idea of gears in general from my very technical training on legos... small gear turning big gear = more torque at a slower speed versus big gear turning small gear = less torque at higher speed.

My blank stare into space though, is how many gears and which feeds into what on the FJ 4x4 system, specifically the 6MT. For gearing with larger tires (like 99.999% of all people re-gearing), I see members swapping for something in the 4.xx. Which gears are these... just the front/rear diff? Or something in the T-case also?

Now, (please dont shoot the messenger), suppose I wanted to go the other way around and drop down from the 3.91? MT to the 3.73 AT gears... is it also just the 2 sets of gears and is literally a swap and bolt on affair?
The manual carrier and the automatic carrier are different. The manual carrier is a 3.91+ ratio that is a thinner ring gear and cannot be simply swapped into a automatic carrier. The automatic carrier is a 3.73- ratio and has a thicker ring gear. The only way to succesfully swap between the two is to switch out the entire third member. Besides you are only talking about maybe 200 rpms difference switching between the two.

If you are looking to upgrade to lower gears without upgrading to an ARB air locker the only option is a 4.88, if you want the 4.56 gears then you will need to install the RD121 air locker. Seems a little confusing but that's how the game works for the FJ Cruiser.

You can look at gearing options on our site if you're interested, we carry the Nitro Gear brand. FJ Cruiser Gearing Options
 

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Here's a basic explanation. The gears they change when going to bigger tire are the differential gears like you said. If you have a 3.73 (like the auto), the drive shafts have to turn 3.73 turns for every turn of the tire. When you go to a bigger tire, you effectively reduce the miles per 1000 rpm. Going from a 32 inch tire to a 33 inch tire will effectively lower your rpms by 32/33. Si if it was revving 1800 at 60, it would now be revving 1745. I know it doesn't seem like a big difference, but it's actually more than than. Your engine is now lower in the power band so it develops less torque and the measured torque is also reduced by the same amount. So if you were putting out say 200 ft lbs at 1800, you might be putting out 190 at 1745, but the actual torque felt by the ground is also reduced by the same factor. So 190 times 32/33 give you 184. The engineers on this forum could explain it better than me. When it comes to manual trannies, they often will give it a slightly higher gear ration 3.93 because they want to give you a better crawl ratio so you can drive very slow in first gear without riding the clutch and will have useable power at a slow speed. In the auto, you have the slip of the torque converter that gets the engine in the power band sooner, and will let you crawl slowly yet still produce enough power to get moving. The number explanation I gave you is multiplied many times at low speed. When a gasoline engine is around idle speed, it may just be producing 10-30 hp and maybe 50 ft-lbs of torque, reducing it by the same amount with bigger tires will have an even bigger effect. At high speed, if you get to a hill and don't produce enough torque to maintain your speed, you just downshift, at very low speed you don't have that luxury. I also have a Chev 350 with a 5 speed manual. Many is the time off-road, I had to ride the clutch just to be able to go as slow as I wanted because of big rocks. Low range first gear just wasn't slow enough. With my auto cruiser, I just have to ride the brakes a little and still have the power to climb rocks easily. That one of the reasons that those with bigger tires see such a dramatic effect on gas mileage. Their engines have to work harder all the time to climb hills, get going, and maintain speed. That plus they have heavier rotational mass (tire weight).
If you go to a bigger tire and re-gear just enough to bring your rev's back to the original number (1800) you still have to deal with the extra rotating weight. So they often re-gear to a bigger number (lower ration like 4.XX) to try to get back to the same performance as stock. Specially important for manual trannies at very low speed.
If you have a manual tranny and don't need the low crawl speeds, you could re-gear to a lower numerical number and gain a couple of mpg's but it would be at the expense of acceleration.
 

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Wow, thanks for all the enlightenment guys! I knew I could count on learning just by asking... especially when I saw all the parts from LT on things needed for a swap and the explanations above are spot on to what Im thinking about.

Not saying I will do this, but does that mean if I were to swap something significantly lower for the diffs, say in the low 3 or high 2 range, then something significantly higher than stock in the TC, Id have: (a) sluggish acceleration due to the gearing on the diffs (b) slow crawl when I switch to the TC LL, (c) high mpg for everyday... but obviously limited by acceleration as mentioned, and (d) kiss towing anything more than my fat self good bye?

Actually, now that I think about it (thanks FJNewb), would it be easier/cheaper to just swap the 6th gear for extended highway (flat) driving and having the rest stock for decent torque/power/crawl?
 

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For gearing with larger tires (like 99.999% of all people re-gearing), I see members swapping for something in the 4.xx. Which gears are these... just the front/rear diff? Or something in the T-case also?
You re-gear the diffs (have to do both front and rear at the same time/same ratio) to change your highway power and speed. You change the transfer case gearing to get lower gearing / higher crawl ratio for the trail. Since all transfer cases are 1:1 in high range, any changes there only affect your low-range gearing like Brian said.
 

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Wow, thanks for all the enlightenment guys! I knew I could count on learning just by asking... especially when I saw all the parts from LT on things needed for a swap and the explanations above are spot on to what Im thinking about.

Not saying I will do this, but does that mean if I were to swap something significantly lower for the diffs, say in the low 3 or high 2 range, then something significantly higher than stock in the TC, Id have: (a) sluggish acceleration due to the gearing on the diffs (b) slow crawl when I switch to the TC LL, (c) high mpg for everyday... but obviously limited by acceleration as mentioned, and (d) kiss towing anything more than my fat self good bye?

Actually, now that I think about it (thanks FJNewb), would it be easier/cheaper to just swap the 6th gear for extended highway (flat) driving and having the rest stock for decent torque/power/crawl?

(a) Depends on what tires you are running....all regearing your diffs does really is influence is where your rpms fall relative to your speed. They do have some influence on your final gear ratio/crawl ration, but not as much as a TC would.

(b) A lower gear ratio (4:1, 10:1,etc. (stock is like 2:1 or something)) will increase your crawl ratio....you'll go slower offroad in LL.

(c) Depends more on your driving style than anything. Lower ratio on your diffs is better for city driving...move torque lower in the RPM band for stop and go, but that doesn't neccessarily help on highway driving....again this all depends on your driving style and tire size.
 
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