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Unread 02-21-2012, 05:27 PM   #1
YJSteveD
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Can I get an explanation on gears?

I see a lot of people talking about gears and whatnot on this forum, and I'd like to find some information about it.
What is it? How does it effect the jeep? What should my jeeps (1989 4cyl 2.5l with 2.5in lift, stock size tires) gears be at?
Thanks. Pretty new to jeeps, had mine since dec. 4th, so I'm still trying to learn about it.

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Unread 02-21-2012, 05:30 PM   #2
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Unread 02-21-2012, 05:32 PM   #3
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that should help you understand how an axle works internally.
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Unread 02-21-2012, 05:38 PM   #4
ThomasYJ
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We're referring to the Ring and Pinion in your front and rear differential; other common terms are "diff", "pumpkin" etc. As for your set up, you should be fine in that you haven't added a huge lift, put bigger tires on, added heavy components such as armor, bumpers/tire carriers/ winches etc. I'm pretty sure your gear ratio is a 4.10:1 being that you have a 4 cyl (someone can step in here to correct me if I'm wrong for some reason). One reason Gears are usually changed out for lower (numerically higher) ratios is to regain drive-ability, both on and off road, after the components mentioned earlier are added which weigh your rig down.
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Unread 02-21-2012, 05:39 PM   #5
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The gears are the ring and pinion in the differentials. Basically they are the only way to get more torque that do not involve major modifications.

The larger the numerical ratio, the lower the gearing. So 4.10 is lower than 3.73 is lower than 3.07. Lower gearing (higher number) gives more torque at the cost of the engine running at a higher rpm.

Larger diameter tires have the effect creating a higher gearing (lower number). This can be compensated for with a lower axle gearing.

The 2.5 liter models had 4.11 gearing in the later year yj's. Not sure about yours. This is the lowest gearing that came stock in yj's. With stock tires I wouldn't change them (assuming 4.11). You would be looking at probably at least 1000 dollars for a shop to regear.


Edit: Wow. I am slow. ^^^^
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Unread 02-21-2012, 06:05 PM   #6
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Wow .. Just reading these few posts on the form I learned so munch .. Keep it up guys/women
There are know bed questions.
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Unread 02-21-2012, 06:05 PM   #7
YJSteveD
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Quote:
Originally Posted by Tiny View Post
Thanks for the link man, awesome video, learned a lot from it. Had no idea what a differential was!
And thanks everyone for the quick responses.
So basically, the higher the weight, the lower the gear ratio should be? And the lower the gear ratio, the higher the RPMs will be?
Man, this is so complicated, I love it. Can't wait to really dive into this and learn about it more in depth!
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Unread 02-21-2012, 06:06 PM   #8
ThomasYJ
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I didnt know chevy invented the differential!!

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Unread 02-21-2012, 06:16 PM   #9
ThomasYJ
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Originally Posted by YJSteveD View Post
Thanks for the link man, awesome video, learned a lot from it. Had no idea what a differential was!
And thanks everyone for the quick responses.
So basically, the higher the weight, the lower the gear ratio should be? And the lower the gear ratio, the higher the RPMs will be?
Man, this is so complicated, I love it. Can't wait to really dive into this and learn about it more in depth!
well along with weight also your intentions/ driving style come into play: rock crawling, racing, Hwy commuting, towing etc, lead foot vs light foot. To a point, a lower gear ratio will give you quicker acceleration, and more torsional force to the wheels with less effort from the engine, but it will lower your top speed. Whereas a higher gear ratio will lessen the torsional force and require more from the engine and damper your acceleration but give you a better overall top speed. (picture a 21 speed bike. you've got lots of torque without a lot of effort on that first gear but soon your legs(engine) are spinning a mile a minute and you're not going very fast. but shift to 21st gear and you'll start out reeeal slow but eventually you'll be movin pretty good. its the same thing )

The ratio of the gear is actually the torque multiplier, this was explained to me by a mechanical engineer. so you're taking the torsional forces of the drive shaft and whatever your diff ratio is, it's actually multiplying it by that number.
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Last edited by ThomasYJ; 02-21-2012 at 06:27 PM.. Reason: re-phrasing
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Unread 02-21-2012, 06:25 PM   #10
YJSteveD
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Originally Posted by ThomasYJ View Post
well along with weight also your intentions/ driving style come into play: rock crawling, racing, Hwy commuting, towing etc, lead foot vs light foot. To a point, a lower gear ratio will give you quicker acceleration, and as mentioned above.. more torque (I'll explain more below) but it will lower your top speed (picture a 21 speed bike. youve got lots of torque on that first gear but soon your legs are spinning a mile a minute and you're not going very fast. but shift to 21st gear and you'll start out reeeal slow but eventually you'll be movin pretty good. its the same thing ) Whereas a higher gear ratio will lessen the torque and damper your acceleration but give you a better overall top speed.

The ratio of the gear is actually the torque multiplier, this was explained to me by a mechanical engineer. so you're taking the torsional forces of the drive shaft and whatever your diff ratio is, it's actually multiplying it by that number.
I'm keeping my jeep on the street until I get everything fixed/do a few more mods, so I'm probably gonna keep my gears where they're at for a while.
So a lower gear ratio would be good for offroading, towing, stuff like that?
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Unread 02-21-2012, 06:34 PM   #11
ThomasYJ
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Originally Posted by YJSteveD View Post
I'm keeping my jeep on the street until I get everything fixed/do a few more mods, so I'm probably gonna keep my gears where they're at for a while.
So a lower gear ratio would be good for offroading, towing, stuff like that?
Smart move! seriously.

And yes, especially for you since you've got the 4cyl., once you add big tires etc you may need to regear. Although, I've seen guys say on here say that their gearing is fine for off-road. its on the road where theyre dying tryin to just maintain speed. make sense?
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Unread 02-21-2012, 06:37 PM   #12
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Quote:
Originally Posted by ThomasYJ View Post
well along with weight also your intentions/ driving style come into play: rock crawling, racing, Hwy commuting, towing etc, lead foot vs light foot. To a point, a lower gear ratio will give you quicker acceleration, and more torsional force to the wheels with less effort from the engine, but it will lower your top speed. Whereas a higher gear ratio will lessen the torsional force and require more from the engine and damper your acceleration but give you a better overall top speed. (picture a 21 speed bike. you've got lots of torque without a lot of effort on that first gear but soon your legs(engine) are spinning a mile a minute and you're not going very fast. but shift to 21st gear and you'll start out reeeal slow but eventually you'll be movin pretty good. its the same thing )

The ratio of the gear is actually the torque multiplier, this was explained to me by a mechanical engineer. so you're taking the torsional forces of the drive shaft and whatever your diff ratio is, it's actually multiplying it by that number.
Right, so the end goal is to match the motor output (makes more power at higher rpms) with your driving needs (low speed crawls vs high speed hwy)

This is almost always a compromise though, and with a small motor like the 2.5 you quickly reach practical limits.
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Unread 02-21-2012, 06:46 PM   #13
YJSteveD
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Quote:
Originally Posted by ThomasYJ View Post
Smart move! seriously.

And yes, especially for you since you've got the 4cyl., once you add big tires etc you may need to regear. Although, I've seen guys say on here say that their gearing is fine for off-road. its on the road where theyre dying tryin to just maintain speed. make sense?
Yeah, totally. Looks like I'll be good for a while.
Thanks everyone!
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Unread 02-21-2012, 06:47 PM   #14
ThomasYJ
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Quote:
Originally Posted by imstillatwork View Post
Right, so the end goal is to match the motor output (makes more power at higher rpms) with your driving needs (low speed crawls vs high speed hwy)

This is almost always a compromise though, and with a small motor like the 2.5 you quickly reach practical limits.
x2

Edit: swap in a V8, tranny and a Ford 8.8 and call it a day! LoL
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Unread 02-21-2012, 06:57 PM   #15
UnFocused
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Hi YJSteve, everyone has given you great information but I noticed there hasn't been a basic explanation, unless I missed it.

If you park your jeep on a flat level surface, release the emergency brake, chock the front wheels and jack up one rear tire off the ground then you can see what's happening. (You don't have to do this, but you can if you want.)

Let's say your rear end is the standard 4:10 gears. If you rotate the wheel one full turn you'll see that the drive shaft rotates 4.1 full turns. If you have 3.08 gears then the drive shaft will rotate 3.08 full turns. Pretty simple.

Now, let's assume that the gear ratio in the transmission relative to engine RPMs is one to one (1:1). This typically the standard ratio for manual transmissions in 4th gear. The 4:10 will cause higher engine RPM (and generally higher horsepower and higher torque) than the 3.08 at any given highway speed, say 65. So, if you're running 65 with a 3.08 your engine (1:1 ratio) is operating at low range in the RPM/HP/Torque curves for the engine and there is less "power" available than higher in the RPM/HP/Torque curves which is made available by the 4:11.

Jeep already selected the optimum ratio for you considering both power and economy. As stated before if you start modifying things (bigger tires, more weight, etc.) then you might have to consider different gear ratios depending on the mods and the HP/Torque curves for the engine.

Here's a link for the curves on the 4.2 litre engine. Sorry I dont have them for the 2.5 (they're entirely different) but this will let you see what's happening:

http://www.clutchkitcenter.com/media...weber/Jeep.gif

Hope this helps, and take care.
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