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Old 01-14-2012, 08:50 PM   #1
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Gear ratio 101

Well I've searched the threads, I've googled and I've read through tons of numbers, calculations and pictorials but none of that gives me a real world understanding of whats going on. If someone would be so kind as to give me a few basics on gear ratio I would be able to sleep good at night. That being said I do know that a 373(3.73:1) gives more torque at the wheel than a 320 and a 410 is even better but what does all that mean? I'm sure there are many new jeep owners that are like me and really want to understand their jeep better and would really appreciate this info.

1) What gear are the stated ratios representing? 1st? and is that in 2hi, 4hi or 4lo?
2) Is the stated ratio the axle, transfer case or transmission? I'm pretty sure it's the axle.
3) Are both axles always the same?

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Old 01-14-2012, 09:00 PM   #2
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When you say 3:73 or 4:10 you are refering to the axle ratio. What that means is the driveshaft turns 3.73 times to make the wheel turn 1 full rotation. The front and rear axle must be the same to use 4wd otherwise the wheels would turn at different speeds. It's kind of like riding a 10 speed bicycle. When you are in first gear it's easier to pedal but you have to pedal really fast to get any speed. When you have a numerically higher axle ratio 4:10 vs. 3.73 the engine will feel more powerful but you will hit your top speed faster and have a lower top speed because the engine will be tunring higher rpms

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Old 01-14-2012, 09:00 PM   #3
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1. Is specific to your transmission and transfer case. Go to wiki, look up your specific model, and then google or wiki that particular model.

2. Axle

3. Yes from the factory, and yes they should stay that way. Some jokers with old 4x4's that want big tires will change the rear ratio and skip the front, especially if they have a big lift and the front driveshaft doesn't fit anyway. With different ratios, you'd be dragging or pushing one set of tires, and causing bucu stress to the drivetrain.
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Old 01-14-2012, 09:01 PM   #4
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let me know if I sound confusing
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Old 01-14-2012, 09:23 PM   #5
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It is not difficult. A 410 gear means it is actually a 4.10 gear ratio. Your driveshaft will turn 4.10 times and your tire will turn once. This gives you greater torque for pulling. With this gear ratio, you pull things better BUT you don't have high top end speed.

If you had another vehicle with 2.73 gear ratio, your driveshaft will turn 2.73 times and your tire will turn once. This makes it harder to tow, but great for highway speeds and economy.

Imagine a dragster... it wants a high gear ratio so it can get "out of the hole" faster... whereas a "salt flat racer" who is trying to set high speed records would want a low gear ratio for higher speeds.

The gears represented are only your axles. Both front and rear axles have to have the same ratio or you will destroy your drivetrain. Your axles have to turn at the same rates or your transfer case will explode.

Now as far as 4Hi or 4 Lo... this has nothing to do with your axle ratios. They are set. However by changing your gearing in the transfer case, you can actually engineer a lower "working" gear ratio for your axles. (It is almost the opposite of adding an overdrive to a vehicle. In an overdrive, the ratio is manipulated for better highway speeds and economy. By using a different transfer case setting, the ratio is manipulated to give you better torque. But both units don't change the physical gear ratios of your axles. They change the "apparent" gear ratios of your axles through proper engineering... kind of like a bicycle does from the pedal sprocket to the rear tire gear. (Think about a 10 speed bike and you just start riding. You have your gears set in an easy ratio where you pedal and pedal, but go nowhere... however, you can go up hill really easy and take off from a dead stop super easy. Now let's say you have the gear lever set in the opposite end... now, taking off from a dead stop is super tough and going up a hill is impossible BUT cruising down a concrete bicycle path is super easy and barely takes any effort. This is what your transfer case does when it goes from 4LO to 4HI.)

It does get confusing sometimes because car people go back and forth between what they mean by "low gears" and "high gears". 4LO just means your vehicle is engaged in the same "gear ratio" that you drive at all the time in normal driving. 4Hi means that your transfer case has dropped to a "higher gear ratio" so that you can pull better in "off road conditions", however your top speed is severely limited... but you can pull anything out (well, within reason...)

On my WWII Willy's, the axle ratio was 4.88 and the engine had 53 horsepower. It went everywhere. My old YJ came from the factory with a 123 horsepower four cylinder and 4.10 gears. It went anywhere... but I kept my suspension stock. If you are going to increase "lift" and put huge tires on, you are probably going to want to regear to 4.88. That will help you better "off road", but the trade off is your top end speed will suffer.

I think this answers all your questions.

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Old 01-14-2012, 10:38 PM   #6
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Quote:
Originally Posted by 1SAS View Post
4LO just means your vehicle is engaged in the same "gear ratio" that you drive at all the time in normal driving. 4Hi means that your transfer case has dropped to a "higher gear ratio" so that you can pull better in "off road conditions", however your top speed is severely limited... but you can pull anything out (well, within reason...)
I think I almost have it but shouldn't what you said be reversed, 4Hi is normal and 4Lo gives you more pulling power.
So the stated gear ratio, for instance I have 3.73:1 for the tow package, that refers to the transfer case gears (with respect to axle revolutions) and that would be 2Hi and 4Hi, and 4Lo gives me a higher ratio for better pulling force?
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Old 01-14-2012, 11:14 PM   #7
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Wait, I think I have it. The stated ratio is what's in the differentials (both front and rear and they have to match) , the transfer case is adjusted to spin the axles at 2 different rates to give you better puling power in 4Lo or better highway speeds in 2/4Hi. Just like a bike, if the rear wheel had one gear and the front sprocket had 2 different sizes.
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Old 01-14-2012, 11:43 PM   #8
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Quote:
Originally Posted by phottomatt
Wait, I think I have it. The stated ratio is what's in the differentials (both front and rear and they have to match) , the transfer case is adjusted to spin the axles at 2 different rates to give you better puling power in 4Lo or better highway speeds in 2/4Hi. Just like a bike, if the rear wheel had one gear and the front sprocket had 2 different sizes.
The stated number is the axle ratio and both axles will be the same. 4Hi has a 1:1 ratio, meaning that the input from tthe transmission will spin the drive shaft at the exact same speed. The input shaft turns once, the driveshaft turns once. 4lo is only to be used when off road when low speed and high torque is necessary, in fact, 4wd should never be used on dry pavement. Anyways, what 4lo does is change the ratio inside the transfer case from 1:1 to 2.73:1 if you have an X, Sport, or Sahara or 4:1 if you have a Rubicon. What this means is that the transmission input will now spin that much faster compared to the front and rear driveshafts. So assuming you have, say, a Sport. When the input shaft from the transmission spins 2.73 times, your driveshaft will only spin once. What this does is reduce your speed while multiplying torque. This is really handy when navigating tight trails, descending steep slick hills, or climbing rocks where slow tire placement is important.

But yes, the bike analogy is pretty accurate. The front sprocket is your "transfercase" and the rear (if it was fixed gear) is your "axle".
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Old 01-15-2012, 06:53 AM   #9
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Photomatt,

You are correct and I made a mistake. Thanks for catching me on it.

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Old 01-15-2012, 07:36 AM   #10
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Awesome......thanks everyone, yeti you were not confusing, and chaos thanks for the transfer case info, I did not know that about the Rubi.
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Old 01-15-2012, 08:17 AM   #11
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Originally Posted by DaisyCutter View Post

3. Yes from the factory, and yes they should stay that way. Some jokers with old 4x4's that want big tires will change the rear ratio and skip the front, especially if they have a big lift and the front driveshaft doesn't fit anyway. With different ratios, you'd be dragging or pushing one set of tires, and causing bucu stress to the drivetrain.

If I remember my history correct, some vehicles came with different size tires from factory, to compensate for the difference in axle ratio, because, the front when converted wasnt the same axle ratio, as the rear, they would be very close, but not exactly. for example a 3.35 & a 3.28, thats just a number pulled from my head, but showing you may not be able to tell a difference by manually spinning the driveshaft & counting revolutions, because it is so close, but still would be dragging a tire if you put same size front & rear.

older 1970's Datsuns with 4Wd, was one example, I said Datsuns, not Nissans, like in the old 620 series trucks. I never kept up with the Nissans, I street raced the old 620, 2wd trucks, as well as the old Datsun 510 cars, with L series engines. & still own & moderate a couple forums, to keep spam free, because of my love for them old Datsuns.

another one which is a laugh, is the old 50's 60's ford tractors, when made with 4WD, used a Dodge Power Wagon front, & they also compensated for axle size with 2 different size tires, normally you should only have engaged in mud, so it wouldnt matter, if a little slip.

there may be others, but thats 2 I know of. anyone know of any others?

hey I couldn't resist, standing watching problem solved, & decided to throw a monkey wrench & see what happens.

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Old 01-15-2012, 08:17 AM   #12
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Originally Posted by The Yeti View Post
When you say 3:73 or 4:10 you are refering to the axle ratio. What that means is the driveshaft turns 3.73 times to make the wheel turn 1 full rotation. The front and rear axle must be the same to use 4wd otherwise the wheels would turn at different speeds. It's kind of like riding a 10 speed bicycle. When you are in first gear it's easier to pedal but you have to pedal really fast to get any speed. When you have a numerically higher axle ratio 4:10 vs. 3.73 the engine will feel more powerful but you will hit your top speed faster and have a lower top speed because the engine will be tunring higher rpms
Cant say it any better than that. ^

Here's a chart

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Old 01-16-2012, 02:16 AM   #13
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Oh yeah, those are approximate RPM's at 55 in the middle of the chart. Sorry, I was half asleep. Orange is right around where you want to be for best fuel economy and power. So, yellow if you drive faster, red if you drive slower. ie, 33's at 70 and 3.73 should be about right.

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