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Ok, there have been a few people with gear ratio questions. I figure I would toss together a very quick post about gears and gear ratios.

We will start with differential gears.

When people talk about the gear ratio of a differential they are talking about the relationship of two gears inside. The ring and pinion (R&P). The ring gear attaches to the carrier and is the larger gear in the diff. The pinion is what sticks out the front of the diff and the driveshaft yoke attaches to.

Here is a picture of a R&P.


So now you know what gear is what and what they look like, we can talk about the ratio. This is simply a correlation between the number of times the driveshaft turns (pinion) to the number of times the wheels turn (ring). A ratio is written as 3:1. The 3 is the number of times the pinion gear turns for every 1 time the ring gear turns. So a 4.10:1 gear ratios means the driveshaft will turn 4.10 times for every one time the wheels turn.

Now, the ratio is calculated by dividing the number of teeth on the ring gear, by the number of teeth on the pinion. This means that as the gear ratio gets lower (numerically higher number), the weaker the gear set becomes (more specifically the pinion). For instance, the ring gear of a 4.10 gears will be smaller and the pinion will be larger than the same gears in a 4.88 gear set.

Now, why regear.

There are a few reasons to regear your differentials. Since tire size will change your final ratio. This is why tire size effects your final ratio and why you will notice a loss in "power" when you move up to larger tires. If you have 30" tires and you travel at 65MPH at 2000RPM's, when you increase your tires size (say to 33"), it will take more engine RPM's to make the tires travel at the same speed. You can change your differential gears to make up for this loss. Same goes for off-road, you can use lower ratio gears to lower your final ratio and allow you to crawl through the woods.

So, for highway cursing use, you want a higher gear ratio so the engine has to work less to maintain a higher rate of speed (this is the purpose of the overdrive in the transmission).

For off-roading, you want a lower gear ratio so that even at high engine RPM's, the wheels will turn slow allowing you to crawl over rocks (this is the purpose of the low range in your transfer case or lower gears in your transmission).

Hope this makes some sense for the folks that previously didn't understand gears and gear ratios. If anyone sees any mistakes, please let me know. If anyone needs any clarification on something, let me know.
 

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:clown: Damn straight Muzik--

--It's just hard for people to realize the effects, of 5th/6th OD gears and AUTO Over Drives, that act the same as putting in HIGHER ration diff gears--

-- :geek: :geek: --JIMBO
 

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Muzikman said:
There are a few reasons to regear your differentials. Since tire size will change your final ratio (remember, for every X number or times the pinion gear rotates, the ring gear rotates once. When you move out to the tires, for every rotation of the ring gear, you will not get a full rotation of the tire. The larger the tire, the more rations of the ring gear you will need to rotate the tire once. This is why tire size effects your final ratio and why you will notice a loss in "power" when you move up to larger tires. If you have 30" tires and you travel at 65MPH at 2000RPM's, when you increase your tires size (say to 33"), it will take more engine RPM's to make the tires travel at the same speed. You can change your differential gears to make up for this loss. Same goes for off-road, you can use lower ratio gears to lower your final ratio and allow you to crawl through the woods.
You were doing real good until right here. When traveling in a straight line your tires and ring gear all rotate at the same rate(RPM). So lets say you are driving along and your ring gear is at 750rpm:

33" * pi * 750 = 78000 "/min = 74 mph
31" * pi * 750 = 73000 "/min = 69 mph

So for the same engine, tranny, and diff rotational speed you have a higher vehicle velocity with the larger tires. This speed is not free, however. The engine driving the larger tires will need to produce more energy to reach that RPM as it is doing more work at that RPM. This model fails to take into account things like friction, rotational inertia, additional mass of the tires, and a host of other factors. Assuming the tire construction is the same in each tire, just 6.5% larger on the 33 vs the 31 then most of that stuff has very little impact.

Another thing to keep in mind is that the engine's efficiency is not linear so despite the fact that it takes more energy to turn the 33's at 750 RPM the engine may work best at that RPM and you could see a reduction in fuel usage.


But I digress.
 

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Okay guys if this is gonna be a sticky it will have to be in layman's terms. Lets just pretend that we are in a vaccume and all of the outside forces have no impact.

Remember when you did all your 101 classes in college??? Whether it be engineering, calculus, statistics, etc. All of the minor things that could affect your conclusion were set to Zero.

No offense to you import... you have some excellent info (and thanks for correcting muzik), but when you start talking like that it tends to intimidate people that dont have a good grasp on what you are talking about. (specifically.... me) LOL
 

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I am gonna read this over and over again so it gets stuck in my head.....this topic is a definite keeper...just like that old video on the diff explanation 101.
 

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Here some more information about ring and pinions:

http://www.truehi9.com/gears2.html

It gets technical in a couple areas but as your knowledge increases so should your understanding of the finer details.

This article focuses on the Ford 9" hi-pinion axles and how they're different from low pinion. Good read.

Great post Jason
 

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So, say in my case, I use my X as my daily driver and am a mild-intermediate off roader. I def want to increase my trail time but actual locations and current physical limitations inhibit me from getting to the next level. Ideally I would want in my X a high pinion gear set w/ a larger ring gear to compensate for my larger tires and frequent highway use?
 

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TXterra37 said:
So, say in my case, I use my X as my daily driver and am a mild-intermediate off roader. I def want to increase my trail time but actual locations and current physical limitations inhibit me from getting to the next level. Ideally I would want in my X a high pinion gear set w/ a larger ring gear to compensate for my larger tires and frequent highway use?
A popular front axle is a high pinion D30, D44 or D60 because of the strength of the ring and pinion. Also, by using a high pinion front axle, you reduce the angle of the drive shaft to the transfer case. That reduces u-joint binding and relieves strain on the yoke.

High pinioned rear axles are rare. There isn't much demand for them because you don't have to deal with binding issues.

Considering the 1 gear ratio, the transfer case gear and 33"/34" tires on a 2nd Gen X, you could swing 4.10's quite nicely but your highway mileage might suffer a bit.
 

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Discussion Starter #16
High Pinion is stronger than low pinion, yes, in most cases. I have no idea what our front diff is, but I am pretty sure our rear is high pinion. Does someone have a pic of our diff to confirm this? If not, I'll look when it stops raining.

It's not to much that you want a larger ring gear as it doesn't work that way, again, it's a relationship between the ring and pinion. You don't change just one, you change both.

I am not sure what type of wheeling you want to get into and what your actual limitations are. I'll tell you now, my limitation is vehicle size and ground clearance. It has nothing to do with my gearing. I however will not throw enough money at the X to lift it the way I would need it to run larger tires to give me more clearance. This is why I own the Jeep.

Even 4.10 gears are going to be a bit low for us, even with 33's, but it's an industry standard size, so if gears are released, this is what I hope for. It will give us a better crawl ratio when on the trail, but it will also effect our fuel efficiency when on the high way.
 

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Discussion Starter #18
As far as I know, ARB is not working on gears. Superior is working on gears and from what Xterra Racing is saying they are working on 4.56 and 4.88. I have emailed them asking about 4.10 gears, but I have not heard back from them. They will also need to release gears for the front diff too or else it will be pointless for us 4x4 folks (front and rear gears have to match or the driveline will bind).
 

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Discussion Starter #20
That is for a locker for the C200 rear.

Lockers and gears, although both part of a diff, are different pieces.

Lockers often replace the carrier, which is what the ring gear attaches to. Some lockers (lunchbox lockers) keep the same carrier and just replace the spider gears inside it.
 
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