Help figuring out gear ratio

OldBuzzard said:
It's the circumference of the tire, where the rubber meets the road, which gives the distance traveled per rotation. That doesn't change, even if the tire is flat.

You had it right farther up, with suggesting the manufacturer's revolutions per mile figure.
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So the rolling circumference doesn't change when the tire is completely flat? Please explain how that is possible. The rolling circumference is a function of the static radius from ground to center of hub. Any other dimension is irrelevant. The manufacturer's revolutions per mile is an approximation, but is less precise.
 
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deadbeat son said:
So the rolling circumference doesn't change when the tire is completely flat? Please explain how that is possible. The rolling circumference is a function of the static radius from ground to center of hub. Any other dimension is irrelevant. The manufacturer's revolutions per mile is an approximation, but is less precise.
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^^^
This.
I have used this method on several vehicles and found it the most accurate.
 
deadbeat son said:
So the rolling circumference doesn't change when the tire is completely flat? Please explain how that is possible. The rolling circumference is a function of the static radius from ground to center of hub. Any other dimension is irrelevant. The manufacturer's revolutions per mile is an approximation, but is less precise.
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Yes, for the radius of a circle. A flat tire is no longer a circle, so the math falls apart. The steel belt doesn't change length just because the tire is flat.
 
deadbeat son said:
So the rolling circumference doesn't change when the tire is completely flat? Please explain how that is possible. The rolling circumference is a function of the static radius from ground to center of hub. Any other dimension is irrelevant. The manufacturer's revolutions per mile is an approximation, but is less precise.
Click to expand...

I only use rev/mile, always has been 100% accurate for me. And I determine that it’s correct by back calculating it based on GPS and known gear ratio to confirm. The calculated diameter based on GPS has always matched the calculated diameter calculated by rev/mile.

As for changing with pressure, it might slightly but I wouldn’t think that would be measurable. Tires on my truck have been as high as 45 (dealer stupidity) and down to 26 (normal road pressure for my size) and there has been no measurable difference in rpm at speed. The tire doesn’t shrink with less pressure, it still has to follow the full circle so I think pressure ends up not really mattering. Just my guess.
 
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I think pressure ends up not really mattering. Just my guess.

I would also expect this to be true for normal operating pressures
 
the more you know (2017_11_20 00_38_12 UTC).jpg
 
A flat tire rolls less distance per wheel revolution than a full tire.

  1. This is the basis of indirect tire pressure monitoring systems that compare average wheel speed on one wheel with that of the other wheels assumed to be covering the same ground. The system wouldn’t work if it weren’t true, and it does work.
  2. The tank or caterpillar track analogy is often used to refute this claim, and this argument is often bolstered by pointing out that the length of a tank tread, and indeed the distance around a steel belted tire, doesn’t change other than a miniscule stretch factor when you move the contact point with the road toward or away from the wheel centre. To understand why the tank track analogy doesn’t work, you have to see that changing the shape of a wheel means the “distance around the outside” is no longer an actual circumference. Remember, what we care about is distance covered per wheel revolution. As a thought experiment, picture a simplified caterpillar/tank with 33cm wheels at each end of the track (i.e. wheel circumference roughly 1m), set 10m apart. That means the total tank track is roughly about 20m long. Are we agreed that if the tank has 1m circumference wheels, then it will cover about 1m forward for each wheel revolution, even though there are two wheels? Now simulate the effect of “inflating” a flat tire by taking a wheel off the tank and putting a huge round tire on it, sized perfectly so the roughly 20m long tank tread stretches around the tire to fit it like a set of traction chains. Now roll this wheel and see how much ground it covers per revolution: since the tread is 20m long, we cover 20m per revolution! Same tread length, 20x the distance per revolution, and no slippage between wheel and tread either way. The small 33cm wheel no longer matters here, since the effective circumference of the wheel is so much bigger.
    “But how can this be?” say tire proponents— the outside of a tire is still the same distance around even when flat! The distance around, though, isn’t a circumference anymore— while a tire tread with a 2m circumference will still have to roll 2m for every 2m the car moves forward, the WHEEL will have to make more revolutions to cover the same 2m when the tire is flat. If you flat the tire so far that you’re running on the rim, then the effective circumference becomes that of the rim, not the tire even though all of the tire tread still goes around for each revolution.

    Another way to think about it is that as the more the tire gets flat, the length of its contact patch gets longer— you can no longer say exactly which part of the circumference of the tire is the contact point on the ground. Say a totally flat tire has a contact patch a foot long: the effective circumference of that tire is what you would get if you cut out the foot long patch and joined the ends together.
 
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LONGJP2 said:
A flat tire rolls less distance per wheel revolution than a full tire.

  1. This is the basis of indirect tire pressure monitoring systems that compare average wheel speed on one wheel with that of the other wheels assumed to be covering the same ground. The system wouldn’t work if it weren’t true, and it does work.
  2. The tank or caterpillar track analogy is often used to refute this claim, and this argument is often bolstered by pointing out that the length of a tank tread, and indeed the distance around a steel belted tire, doesn’t change other than a miniscule stretch factor when you move the contact point with the road toward or away from the wheel centre. To understand why the tank track analogy doesn’t work, you have to see that changing the shape of a wheel means the “distance around the outside” is no longer an actual circumference. Remember, what we care about is distance covered per wheel revolution. As a thought experiment, picture a simplified caterpillar/tank with 33cm wheels at each end of the track (i.e. wheel circumference roughly 1m), set 10m apart. That means the total tank track is roughly about 20m long. Are we agreed that if the tank has 1m circumference wheels, then it will cover about 1m forward for each wheel revolution, even though there are two wheels? Now simulate the effect of “inflating” a flat tire by taking a wheel off the tank and putting a huge round tire on it, sized perfectly so the roughly 20m long tank tread stretches around the tire to fit it like a set of traction chains. Now roll this wheel and see how much ground it covers per revolution: since the tread is 20m long, we cover 20m per revolution! Same tread length, 20x the distance per revolution, and no slippage between wheel and tread either way. The small 33cm wheel no longer matters here, since the effective circumference of the wheel is so much bigger.
    “But how can this be?” say tire proponents— the outside of a tire is still the same distance around even when flat! The distance around, though, isn’t a circumference anymore— while a tire tread with a 2m circumference will still have to roll 2m for every 2m the car moves forward, the WHEEL will have to make more revolutions to cover the same 2m when the tire is flat. If you flat the tire so far that you’re running on the rim, then the effective circumference becomes that of the rim, not the tire even though all of the tire tread still goes around for each revolution.

    Another way to think about it is that as the more the tire gets flat, the length of its contact patch gets longer— you can no longer say exactly which part of the circumference of the tire is the contact point on the ground. Say a totally flat tire has a contact patch a foot long: the effective circumference of that tire is what you would get if you cut out the foot long patch and joined the ends together.
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That makes a lot of sense to me visually
 
SouthernTJ2000 said:
I'm starting to think this as well. But why would anyone re-gear to a 3.55?!?!?! lol.
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Maybe broke one of the original axles and best they could find as a replacement was a set of 97 axles that were in 3.55. Or maybe they bought another Jeep, yours had 3.73 Dana 44 originally, they wanted to take that wth them and so to make it work they had to buy a set of axles, that once again, they could only locate a 3.55 pair locally. No telling really.

Have you run the VIN through the FCA Equipment Listing to see what you had originally?

I will say, I'd do the rpm gps comparisons in 4th. It will be a little bit more accurate because the rpm doesn't change as much at different speeds in 4th. Might help determine what you really have.

And I would suggest pulling the diff cover at some point to actually physically confirm the ratio on the gears.
 
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machoheadgames said:
Maybe broke one of the original axles and best they could find as a replacement was a set of 97 axles that were in 3.55. Or maybe they bought another Jeep, yours had 3.73 Dana 44 originally, they wanted to take that with them and so to make it work they had to buy a set of axles, that once again, they could only locate a 3.55 pair locally. No telling really.

Have you run the VIN through the FCA Equipment Listing to see what you had originally?

I will say, I'd do the rpm gps comparisons in 4th. It will be a little bit more accurate because the rpm doesn't change as much at different speeds in 4th. Might help determine what you really have.

And I would suggest pulling the diff cover at some point to actually physically confirm the ratio on the gears.
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Yeah, I did the VIN pull a week or so ago. Its actually what got me to wondering more about the gear ratio as it calls out 3.07 on the build sheet. Both axle tags call 3.07 as well. I'll probably pull the covers this weekend and add the Lube Locker gaskets. Honestly, I'm going to re-gear to 4.56, so all of this is just for my basic/general/historical knowledge of the Jeep. Thanks for the input.



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SouthernTJ2000 said:
Yeah, I did the VIN pull a week or so ago. Its actually what got me to wondering more about the gear ratio as it calls out 3.07 on the build sheet. Both axle tags call 3.07 as well. I'll probably pull the covers this weekend and add the Lube Locker gaskets. Honestly, I'm going to re-gear to 4.56, so all of this is just for my basic/general/historical knowledge of the Jeep. Thanks for the input.



View attachment 517450

View attachment 517452
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4.88 would be the right ratio for the 5-speed and 33’s (or 32.5, whatever).
 
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JMT said:
4.88 would be the right ratio for the 5-speed and 33’s (or 32.5, whatever).
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So it'll see 80% pavement pounding and 20% wheeling. I don't have many wheeling options around me and the two that are "close" are an hour away. Both were a 4x4 Park at one time, however they are basically a SxS/4 wheeler on tractor tire mud holes now, with maybe an hour of trails for full size vehicles. All the good parks are at least 2.5+ hours from me. So I want to be able to jump on the HWY and cruise at 70-75. Would you still suggest 4.88?
 
machoheadgames said:
I'd definitely second the idea to go with 4.88 in lieu of 4.56, if it were mine.
Click to expand...

So it'll see 80% pavement pounding and 20% wheeling. I don't have many wheeling options around me and the two that are "close" are an hour away. Both were a 4x4 Park at one time, however they are basically a SxS/4 wheeler on tractor tire mud holes now, with maybe an hour of trails for full size vehicles. All the good parks are at least 2.5+ hours from me. So I want to be able to jump on the HWY and cruise at 70-75. Would you still suggest 4.88?
 
SouthernTJ2000 said:
So it'll see 80% pavement pounding and 20% wheeling. I don't have many wheeling options around me and the two that are "close" are an hour away. Both were a 4x4 Park at one time, however they are basically a SxS/4 wheeler on tractor tire mud holes now, with maybe an hour of trails for full size vehicles. All the good parks are at least 2.5+ hours from me. So I want to be able to jump on the HWY and cruise at 70-75. Would you still suggest 4.88?
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I would, needing to drive a few hours is not a problem. 3000 at 75 is nice. Have to remember there were autos that were more like 3300-3400 at 75 from the factory, and they were fine.
 
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SouthernTJ2000 said:
So it'll see 80% pavement pounding and 20% wheeling. I don't have many wheeling options around me and the two that are "close" are an hour away. Both were a 4x4 Park at one time, however they are basically a SxS/4 wheeler on tractor tire mud holes now, with maybe an hour of trails for full size vehicles. All the good parks are at least 2.5+ hours from me. So I want to be able to jump on the HWY and cruise at 70-75. Would you still suggest 4.88?
Click to expand...

The gearing is mainly for highway driving, but also a benefit in crawl ratio off-road. Those rpm’s at 70-75mph and 488’s will be nice
 
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